What is your idea of an "Alias" I see it used as a term here all the time, but there is not actually a "thing" called an alias in email, so like most old wives tales. Definitions differ.

Are you using the Microsoft one https://support.microsoft.com/en-au/office/add-or-remove-an-email-alias-in-outlook-com-459b1989-356d-40fa-a689-8f285b13f1f2

Notice they say it is an email address. what they do not say, is that if you use a mail client to send email for one of these "alias" addresses that they will strip the information inserted by the mail client and replace it with that of the primary registered alias as the message goes through their mail servers.

You are far netter off using it as a real email address and using the appropriate sign in information for that address in the SMTP as that avoids the whole stripping of name and email address data. I will point out that they do not do this is you use their web page interface, only mail clients.

So you need to have an outgoing server(SMTP) entry in account settings for each and every one of these ALIAS addresses and not use them as an Alias as such.

Hi,

Thanks for your reply. Yes, they are aliases attached to my main account as per the MS definition. I can send emails from Win10 mail from each of those aliases and they work fine. I can also send emails from each of those aliases from the MS web interface, but not from Tbird.

If I make one of those aliases primary, then I can send emails from that one.

I do have an outgoing server IMAP SMTP entry for each of those accounts, outlook.office365.com on port 993.

If it works in Win 10 mail or from the web interface, why doesn't it work in Tbird?

It also works in New Outlook.

Regards,

Steve

You are far better off using it as a real email address and using the appropriate sign in information for that address in the SMTP as that avoids the whole stripping of name and email address data. I will point out that they do not do this if you use their web page interface, only mail clients. So you need to have an outgoing server(SMTP) entry in account settings for each and every one of these ALIAS addresses and not use them as an Alias as such.

This is exactly what I do.