For @custom-domain.ext mailbox format go here:
Aruba.it custom domains IMAP, POP3, SMTP settings

POP3

Server: pop3.aruba.it
SSL: true-implicit
Port: 995 (default)
User :  email address (e.g.: name@aruba.it or name@technet.en)

using (Pop3 client = new Pop3())
{
    client.ConnectSSL("pop3.aruba.it");
    client.UseBestLogin("user@aruba.it", "password");

    // ...

    client.Close();
}

Next step: receive emails using POP3.

IMAP

Server:  imap.aruba.it
SSL: true-implicit
Port: 993 (default)
User: email address (e.g.: name@aruba.it or name@technet.en)

using (Imap client = new Imap())
{
    client.ConnectSSL("imap.aruba.it");
    client.UseBestLogin("user@aruba.it", "password");

    // ...

    client.Close();
}

Next step: receive unseen emails using IMAP.

SMTP

Server:  smtp.aruba.it
SSL: true-implicit
Port:  465 (default)
User: email address (e.g.: name@aruba.it or name@technet.en)

using (Smtp client = new Smtp())
{
    client.ConnectSSL("smtp.aruba.it");
    client.UseBestLogin("user@aruba.it", "password");

    // ...

    client.Close();
}

For @aruba.it and @technet.it mailbox formats go here:
@aruba.it and @technet.it IMAP, POP3, SMTP settings

POP3

Server: pop3s.aruba.it
SSL: true-implicit
Port: 995 (default)
User :  email address (e.g.: name@aruba.it or name@technet.en)

using (Pop3 client = new Pop3())
{
    client.ConnectSSL("pop3s.aruba.it");
    client.UseBestLogin("user@custom-domain.ext", "password");

    // ...

    client.Close();
}

Next step: receive emails using POP3.

IMAP

Server:  imaps.aruba.it
SSL: true-implicit
Port: 993 (default)
User: email address (e.g.: name@aruba.it or name@technet.en)

using (Imap client = new Imap())
{
    client.ConnectSSL("imaps.aruba.it");
    client.UseBestLogin("user@custom-domain.ext", "password");

    // ...

    client.Close();
}

Next step: receive unseen using IMAP.

SMTP

Server:  smtps.aruba.it
SSL: true-implicit
Port:  465 (default)
User: email address (e.g.: name@aruba.it or name@technet.en)

using (Smtp client = new Smtp())
{
    client.ConnectSSL("smtps.aruba.it");
    client.UseBestLogin("user@custom-domain.ext", "password");

    // ...

    client.Close();
}

First you need to register your application in Azure Portal.

Here’s a detailed guide how to do that:
https://docs.microsoft.com/en-us/azure/active-directory/develop/quickstart-register-app

Remember to add authentication entries (localhost is needed for .net core):

Then you need to apply correct API permissions and grant the admin consent for your domain.

In the API permissions / Add a permission wizard, select Microsoft Graph and then Delegated permissions to find the following permission scopes listed:

• offline_access
• email
• IMAP.AccessAsUser.All
• POP.AccessAsUser.All
• SMTP.Send

Remember to grant admin consent.

Use Microsoft Authentication Library for .NET (MSAL.NET) nuget package to obtain an access token:

https://www.nuget.org/packages/Microsoft.Identity.Client/

var pcaOptions = new PublicClientApplicationOptions
{
    ClientId = "Application (client) ID",

    TenantId = "Directory (tenant) ID",
    // -or-
    // for @outlook.com/@hotmail accounts instead of setting TenantId use:
    // AadAuthorityAudience = AadAuthorityAudience.PersonalMicrosoftAccount, 

    RedirectUri = "https://login.microsoftonline.com/common/oauth2/nativeclient"
    // RedirectUri = "http://localhost" // for .net core
};

var pca = PublicClientApplicationBuilder
    .CreateWithApplicationOptions(pcaOptions)
    .Build();

var scopes = new string[] 
{
    "offline_access",
    "email",
    "https://outlook.office.com/IMAP.AccessAsUser.All",
    "https://outlook.office.com/POP.AccessAsUser.All",
    "https://outlook.office.com/SMTP.Send",
};

In addition, you can request for offline_access scope. When a user approves the offline_access scope, your app can receive refresh tokens from the Microsoft identity platform token endpoint. Refresh tokens are long-lived. Your app can get new access tokens as older ones expire.

Now acquire the access token and user email address:

var authResult = pca.AcquireTokenInteractive(scopes).ExecuteAsync().Result;

string user = authResult.Account.Username;
string accessToken = authResult.AccessToken;

Finally you can connect to IMAP/POP3/SMTP server and authenticate:

using (Imap client = new Imap())
{
    client.ConnectSSL("outlook.office365.com");
    client.LoginOAUTH2(user, accessToken);
 
    client.SelectInbox();

    // ...

    client.Close();
} 

As this is fairly new feature for Exchange/Office365, here are some useful links:

https://docs.microsoft.com/en-us/exchange/client-developer/legacy-protocols/how-to-authenticate-an-imap-pop-smtp-application-by-using-oauth

https://stackoverflow.com/questions/29747477/imap-auth-in-office-365-using-oauth2

https://stackoverflow.com/questions/43473858/connect-to-outlook-office-365-imap-using-oauth2

https://stackoverflow.com/questions/61597263/office-365-xoauth2-for-imap-and-smtp-authentication-fails

Extension is available for other Visual Studio versions:

• GoToTest macro for VisualStudio 2008
• NavigateToTest VS2010 extension
• NavigateToTest VS2012 extension
• NavigateToTest VS2013 extension
• NavigateToTest VS2015 extension
• NavigateToTest VS2017 extension
• NavigateToTest VS2019 extension

You can download the extension here:
NavigateToTest Visual Studio 2019 extension

Here’s the latest version that supports Visual Studio 2019.

Extension is convention based. It matches ClassName file with ClassNameTest or ClassNameTests and vice-versa, so you can easily navigate to the test file and back.

Here are some screenshots:

Here’s the toolbar name, in case it is not added automatically:

You can download the extension here:
NavigateToTest Visual Studio 2019 extension

Here is the list of some helpful links regarding IMAP and POP3 protocols in Exchange 2019:

Enable IMAP4 in Exchange 2019

Enable POP3 in Exchange 2019

POP3 and IMAP4

Public folders in Exchange 2019
Public folders and IMAP

Shared mailboxes in Exchange 2019
Accessing shared and delegated mailboxes