Office 365 enable IMAP/POP3 and SMTP access

First log in to Microsoft 365 admin portal at https://admin.microsoft.com/ as an administrator, go to Org settings screen and find Modern authentication entry:

Check ‘Turn on modern authentication…‘ for OAuth flows.

Check IMAP, POP3 and SMTP for App passwords flows.

Then go to Users screen:

Select an user and on the Mail tab click Manage email apps

Check IMAP, Pop and Authenticated SMTP to turn on the protocols for this account

Have in mind it takes 20-30 minutes for the changes to take effect.

AD configuration

In your Active Directory, make sure Enable Security defaults is set to No:

Make sure there are no Conditional Access | Policies defined in your AD:

For MFA enabled/enforced accounts you must create and use App passwords or OAuth flows.

For other accounts – you should be able to log in using your primary password (remember MFA needs to be disabled):

using (Imap imap = new Imap())
{
    imap.ConnectSSL("outlook.office365.com");
 
    imap.UseBestLogin(
        "AdeleV@limilabs.onmicrosoft.com",  
        "password");
 
    imap.SelectInbox();

    List<long> uids = imap.Search(Flag.Unseen);
 
    foreach (long uid in uids)
    {
        IMail email = new MailBuilder()
            .CreateFromEml(imap.GetMessageByUID(uid));
        string subject = email.Subject;
    }
 
    imap.Close();
}

Exchange administration

You can find the same mailbox/user settings through Exchange administration screens:

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