You must have separate smtp servers for each account, and then match each account with the correct server:

https://support.mozilla.org/en-US/questions/1350593

The outlook smtp should be smtp.office365.com, 587, STARTTLS, OAuth2, but have different User Names (email address). Give each one a distinct Description.

Thank you. I didn't read the article but I already have separate "User Names" <email addresses> for each account. The email will send just fine through the proper account, if that account is designated as the default. Let's say I am wanting to send on account1 but the default is account2. The error states I cannot send an email addressed to account1 while attempting to send it through account2. As soon as I change the default to account1, the email sends fine.

The 'default' account is not the issue. As @sfhowes states, the important issue is having the proper SMTP server set for each account. It appears that you do not. An easy check is to change the Description field for each SMTP server to be the email address. Then, check, the account pane, to see if that appears in the bottom line that shows the SMTP server.

I am sorry for the confusion. I already have the servers setup in TB, I was referring to them as accounts since it made more sense to me. It seems odd when you create an account in TB, it also creates the incoming and outgoing records for the server settings but refers to the outgoing server settings as a "server" and the incoming server settings as "server settings".

Anyway, since they are all outlook.com addresses, they all use smtp.office365.com with the settings sfhowes stated. I have individual descriptions for each server record and the user names are the email addresses of which I am sending from.

So...when I choose to send from an email address of which I have an outgoing server record, TB will not send the email and instead produce the error. Once I have changed the outgoing default server to match the email address I am sending from within the email itself, TB then sends it without any issue.

That is how it must work. the SMTP server check to be sure that the sender is a customer of the email provider. Once you set them properly, you're never prompted again.

I am not being prompted for anything, I either receive the error or it sends based on what I explained earlier. Are the server descriptions for each server record required to also be the email address. They are currently not but each are unique.

The Description field is there to make it easier to point the accounts to the correct smtp server. You should not be changing smtp settings on a per message basis if you have separate smtp servers, and the accounts are set to send on the smtp with a matching User Name.

Unfortunately, that is sort of what is happening except it is not every message. Only when I attempt to send from an address other than the default send server. I can provide snapshots.

Did you carefully check each account? If screenshots are done, do these: - screenshot of the SMTP pane for each SMTP server - screenshot of each account pane (the one that shows SMTP server on bottom row) Seeing those will clarify the situation.

Please let me know if you need anything else.

Now, do screenshots of the account pane for each, the one showing the SMTP server on bottom line. Thank you.

Thank you. That solved it. I did not realize you guys were talking about the actual account settings. I kept going to the outgoing server settings and overlooked that there was an SMTP setting in the account settings. All accounts were set to the default account so I changed them to be their respective correct server and all is good.

We're pleased that all is well. :)