Reinstalling Thunderbird won't make a difference. Using another mail client might, not because there is anything wrong with Thunderbird in this regard, but because you'd have to set up the account again and that could reveal some mistake you made when setting up the account in Thunderbird. Or it might still not work in Outlook for you even though it does for them, which would be informative as well… I find funny these error messages where they tell you there is a problem with the sender but you should check the recipient, don't really understand that… But anyway, a possible cause of this problem could be that the message's "From" doesn't match the username used to authenticate with the SMTP server, for example. It would help if you post a screenshot of your outgoing server settings, in case there is something you're missing there. And you may want to take a look at the following article in the meantime: https://support.mozilla.org/kb/cannot-send-messages