In your Microsoft account - logged on via browser.... How many email addresses have you got registered in the account ? Obviously, you have an @outlook.com email address, but are there any others ?

Microsoft now deem any email addresses whether hotmail or outlook etc or gmail set up to synch or other domain name email addresses - they are called 'aliases' email addresses to the Microsoft account.

They are all supposed to use the same password as the Microsoft Account Password. You can choose to set one account as the 'primary' account. Whatever you set up as 'primary' is now the email address used to 'send' email via a third party client using smtp. If you typically use a outlook.com email address is that outlook.com email address set up as the 'primary' alias or is another 3rd party email address set up as 'primary'?

Also whilst in Microsoft account / outlook.com email Top right where you click to see account and options to sign out - you might see that weird email address.

please check to see if any rules have been created to forward any emails.

Hi @Toad-Hall,

Thank you for your help. I've checked and I have no aliases with my outlook email address. It is my primary and only email address with my Microsoft account.

I also do not see the weird email address in my Microsoft account or on outlook.com, and there are no rules created to forward emails.

Thank you again for your help.

I found this image which explains where the user found that weird name for their outlook account.

The same issue as you was reported on microsoft forum - actually there's a few of these cases reported. https://answers.microsoft.com/en-us/outlook_com/forum/all/improperly-displayed-my-e-mail-address-in/ee8fedd2-b0f6-4d5e-bc27-34491006af09 A user in that above link posted an image showing where they saw the data - You can use that link but I've posted the image below.

Also this link: https://answers.microsoft.com/fr-fr/outlook_com/forum/all/default-sct-msonline-outlook-templatetenant-%c3%a0/8673ba2e-78a2-4972-9b6b-d0522f6cfedb?page=5

This error you see is not something caused by Thunderbird.

EXCHANGE+20ADMINISTRATIVE+20GROUP - this sounds like it's got something to do with an exchange account and using a template.

Are you using a VPN on computer ? Many Anti-Virus now offer VPNs and in some cases they have been auto enabling them causing a lot of various problems for anyone using third party email clients as servers do not like your IP being hidden or looking like it's in another country. They think you might be a hacker. Please check you do not have a VPN operating. If you do then switch it off.

What server settings are you using in Thunderbird ? Post image showing Account Settings > Server SEttings and also the Outgoing Server Settings.

When you start up computer - assuming you are using a Windows OS - do you get asked to logon to a Microsoft account OR do you logon to User Account ?

If you use a Microsoft Account - do you logon using that same outlook.com address ?

Based on those Microsoft forum posts, it looks like I may have to contact Microsoft support to resolve my issue. Private messages were exchanged, which supposedly solved the user's jumbled email address.

The server settings are shown in the attached images.

I do not use a VPN. But I do logon to my Windows PC with the same outlook.com email that is causing issues.

Thank you again @Toad-Hall for your dedication in helping me!