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Winmail.dat attachment from Outlook. The definitive guide

You use Outlook and send emails with attachments. A recipient of an email tells you that he cannot read the attachment: instead of the attachment there is a file named winmail.dat that he cannot open. The recipient of your email is probably not using Outlook but another email client. It is interesting that if you send an email to another recipient who uses the same email client as the first, same version, he can see and read the attachment correctly.

The first consideration is that the problem “is in you” and not your mail recipients. You need to fix the problem on your Outlook.

Solution 1: send emails as Html

For the first solution you will find hundreds of articles on the internet, we report it here for completeness. You need to set Outlook to send emails in HTML format. In Outlook:

File -> Options

Select “Mail” on the left. You have to make 2 variations.

  1. Under the Compose messages section, change the “Compose messages in this format” dropdown box to HTML
  2. In the “When sending messages in Rich Text format to Internet recipients”, ensure you select either Convert to HTML format

Restart Outlook

Outlook from several versions already has these default settings, so you didn’t need to make these changes. In fact, if that were the problem, it would not explain how some of your recipients are successfully receiving attachments. You haven’t solved the problem.

The definitive solution. The problem of self-completion

If you have correctly configured Outlook as above, here is the definitive solution. The problem is in the auto-completion of the email address. The recipients of your emails who have this problem are surely in memory in the Outlook auto-completion. Auto-completion is that function that is activated while you write the email address in a new email: while you type the recipient’s address, a drop-down menu appears showing the last used addresses. If you use the proposed address, your recipient will receive the winmail.dat file.

If, on the other hand, you write the email address without selecting the address proposed by the auto-completion, your recipient will receive the right attachment.

Let’s recap. The winmail.dat problem affects only some of your recipients; they are recipients who do not use Outlook and have the e-mail address stored in the Outlook memory which appears with auto-completion.


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Outlook for Exchange online: Change the default address book

Exchange online Outlook accounts has the Global Address List (GAL), as a default address book: the one that contains corporate accounts. To set the user’s address book (conatcts) as default, synchronized on Outlook online, you need to:

  1. open Outlook
  2. at the top of the main page there is an icon-button “Address Book”.
  3. In the window that appears select “tools – options”.
  4. select “Start with contact folder”
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Remove Thunderbird from default email client on Windows


Even though Outlook is set as the default email client in Windows, when you use that program’s “Send as email” function it brings out a Thunderbird dialog box instead of Outlook.


Open regedit. (‘Start’ > Run > type regedit)

Check registry key: HKEY_CURRENT_USER\Software\Clients\Mail

right side double click on ‘Default’ Enter Microsoft Outlook in ‘Value Data’

Windows 10 may have some variants

  1. HKEY_CURRENT_USER\Software\Clients\Mailto
  2. HKEY_CLASSES_ROOT\mailto\shell\open\command
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Register a web application with Azure AD Portal App Registration to connect to a Microsoft 365 tenant

PowerShell Limits

Through Powershell it is possible to connect to a Microsoft 365 tenant to perform operations on users, groups and any other element of the tenant. When you use this tool, Powershell presents you with the mask for entering your account and password. You can write accounts and passwords directly in the Powershell script but it would be a serious security compromise.


An alternative is to build a software that connects directly to the Tenant through customized keys present in the Tenant itself. In other words, it is necessary to communicate to the Tenant that there is a certain application that is authorized to access the Tenant. Furthermore, for each operation that you want to perform on the Tenant it is necessary to specify the appropriate permissions. To create these applications, we recommend that you follow the excellent tutorial “.Net Core console application for calling Microsoft Graph“.  This post proposes the images present in the previous tutorial only to specify how the application must be prepared on the Microsoft Tenant.

Register a web application with Azure AD Portal App Registration

Open a browser and navigate to the Azure Portal. Login using your account. Select the resource “Azure Active Directory”. On the left side menu, select “App regitstration”. Click New registration from the current page.

On the Register an application page, specify the following values:

  • Name = Name of your Application
  • Supported account types
  • Redirect URI
    • Type = Web
    • Value = https://localhost:8080   (*)

(*) The Redirect URI value must be unique within your domain. This value can be changed at a later time and does not need to point to a realy hosted URI.

It is now necessary to store 2 values that will be used in your application:

  • Application (client) ID
  • Directory (tenant) ID

Certificates & secrets

Click Certificates & secrets.

  1. Click New client secret.
  2. On the Add a client secret dialog, specify the following values:
    • Description = Your secret’s description
    • Expires = In 1 year (for example)
  3. Click Add.

After the screen has updated with the newly created client secret copy the VALUE of the client secret. This secret string is never shown again, so make sure you copy it now.

API permissions

Click API permissions.

  • Click Add a permission
  • On the Request API permissions panel select Microsoft Graph.

  • Select Application permissions.

Now you have to choose between the permissions to authorize your app. For example, to create an application to read alla information about Tenant’s users, in the “Select permissions” search box type “User”.Select User.Read.All from the filtered list. At the end, on the API permissions content blade, click Grant admin consent for the Tenant.

Summary of the data necessary for the application

Let’s see what data your application needs to connect and operate on the Microsoft Tenant.

  • applicationId = “xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx”;
  • applicationSecret = “xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx”;
  • tenantId = “xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx”;
  • redirectUri = “https://localhost:8080”;
  • domain = “”;


  • User.Read.All : Read all users’ full profiles
  • User.ReadWrite.All : Read and write all users’ full profiles
  • Group.ReadWrite.All : Read and write all groups
  • Notes.ReadWrite.All : Read and write all OneNote notebooks


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Set up an Office 365 account on GMail

This article was born from the difficulties encountered in setting up a Microsoft Office 365 mail account on the Gmail mail client on a smartphone. But

Reading Office 365 in Gmail is not recommended.

In the Gmail add account screen, the most obvious attempt is to add the account using “Exchange and Office 365”.

You will see the mailbox added correctly and working but we have encountered problems: in some accounts on Gmail you do not see all the mail folders in the mailbox.


The solution is to perform an installation in imap.

Install the account using “Other”:

Once you have entered your email account, the “Manual Configuration” button appears.

Push it. The procedure will ask you in sequence:

address of the mail arrival server:
address of the mail receiving server:

The procedure does not require the specification of ports. For the sake of completeness, we recall all the parameters of the Microsoft Office 365 imap and smtp.

IMAP settings – Server name: – Port: 993 – Encryption method: SSL

SMTP settings – Server name: – Port: 587 – Encryption method: TLS or STARTTLS


By doing this you deactivate the synchronization of address book, calendar and email. To solve it, simply go to the Outlook app, in the account and reactivate the synchronization.

But remember that the best thing for Office365 is to use the Outlook app on smartphones. Reading Office 365 in Gmail is not recommended.


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Email – how to move your address book from Thunderbird to Outlook

  1. export the Thunderbird address book in .CSV format without special configurations
  2. while importing the .CSV file into Outlook, after choosing the “Contacts” folder in which to import the contacts, click on “Map Custom Fields“, select the fields of the .CSV file and associate them with the fields of the Outlook address book.
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Outlook – I don’t see emails imported with a .pst file


You need to move emails from one account or personal folder to a different IMAP account. You are using Outlook. You exported the “source” emails to a pst file and imported the file with Outlook to the “destination” account. Everything seems to have been done correctly because you see the folders of the old emails on the new account, however, inside the folders, you don’t see the emails.

Don’t be alarmed! The emails are there ….. but they are hidden


In Outlook, click on a folder where you don’t see the content. At the top click on View – Change View. In the window that appears click on “IMAP Messages“. If you have many folders in the same window,  you have the “Apply current view to other mail folders” button.

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Problemi delle caselle email molto grandi

La posta elettronica è cambiata molto negli anni: da semplice mezzo di comunicazione è diventato il nostro “contenitore” preferito di informazioni. Conserviamo tutto nella nostra cassetta postale: comunicazioni, contratti ordini, fatture, etc. Queste informazioni devono sempre essere accessibili ovunque noi siamo: non possiamo più vederle solo dalla nostra postazione di lavoro ma anche attraverso il nostro smartphone.

Le aziende e gli utenti chiedono ai Provider caselle email di dimensioni sempre maggiori. In azienda l’email è e rimane il principale mezzo sia di comunicazione ma, oggi, anche il principale strumento per archiviare i file. Conserviamo tutte le email che riceviamo con allegati di qualsiasi dimensione; conserviamo anche tutte le email che inviamo anche se contengono allegati che abbiamo prelevato dal nostro stesso computer.

Il motivo è che inviare un file via email è semplice, veloce e comodo, inoltre quel file rimarrà per sempre nella casella dell’utente, che così potrà averlo sempre a portata di mano. Sarebbe stato meglio spostare il file su un server di rete o utilizzare in azienda servizi di cloud storage , ma l’email rimane il metodo più immediato.

L’altro motivo per cui le email crescono sempre è che gli utenti non sentono il bisogno di fare spazio nella propria casella email, magari cancellando i messaggi che non servono più o semplicemente svuotando il cestino.

Ma quali sono i problemi di una caselle email di grandi dimensioni?

Il primo sono i tempi di avvio del proprio programma per leggere le email, ad esempio Microsoft Outlook diventa sempre più lento man mano che la caselle email diventa più grande.

Un altro problema è che non tutti i fornitori di web e email hosting forniscono caselle email di grandi dimensioni, molti non vanno oltre pochi GB di spazio per casella email.

Inoltre una casella email di grandi dimensioni diventa onerosa da gestire anche lato server il che la rende lenta nell’accesso e nelle ricerche.

Quali sono invece i limiti di dimensione che una casella email può raggiungere?

La documentazione ufficiale di Microsoft Outlook dichiara che le prestazioni di Outlook iniziano a peggiorare non appena il file di archivio delle email OST supera i 10GB. Superato i 25GB i problemi di lentezza e blocchi di Outlook aumentano.

Arrivati a 50GB Outlook smette di funzionare in quanto questa è la dimensione massima che può raggiungere un file PST, ed è necessario eseguire delle operazioni di manutenzione, con conseguente fermo del lavoro dell’utente.

Lato server, ovvero per il fornitore del servizio di email hosting, non ci sono limiti veri e propri di dimensione ad una casella email, ma potrebbero esserci dei problemi nell’indicizzazione e nell’archiviazione di caselle email molto grandi.

il nostro servizio di gestione email

Mecdata in collaborazione con il partner Qboxmail offre un servizio di gestione email professionale:

  • Casette postali di diverse dimensioni/costi per rispondere a qualsiasi esigenza : 4GB, 8GB, 25GB, 50GB
  • Cassetta postale sempre aggiornata e uguale in ogni istante su ogni vostro dispositivo : computer, notebook, tablet, smartphone
  • Webmail, sviluppata ed ottimizzata per gestire caselle di grandi dimensioni

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