I routinely use two SMTP servers with authentication. Passwords are stored in TB so I'm not prompted for them when sending.
Recently, on a few occasions, a box has popped up telling me that sending has failed and inviting me to retry or enter the password. Knowing that the password is correct, I've generally selected retry - which sometimes works and sometimes doesn't (if it doesn't I usually abandon the attempt for a while).
I had assumed that this was a transient authentication issue with the SMTP server (and it may well be) but the odd thing is that afterwards if I go into my account settings (Tools -> Account Settings -> Outgoing Server (SMTP)) and look at the list of SMTP servers I have defined in TB, the only one shown on the list is the one via which I've just experienced the failure. I have five servers defined, but the other four are missing.
If I shut down and restart TB, the settings are back as they should be (with no intervention from me).
All we can really say about this is that something - under certain circumstances - appears to be preventing TB from reading the SMTP server settings (which may also account for the failure to send).