Helping people with computers... one answer at a time.
OK, that was me, this morning, asking the question. I was tearing my hair out trying to send an email to a members-only discussion list, and no matter that I explicitly told Outlook to use the account that I use on that list, it insisted on using one of the others. Silently, of course.
The answer surprised me.
Outlook 2003, and previous versions, can be configured to send and receive email using many different accounts on any number of mail servers. I have several accounts myself. In fact a friend was updating my information in her address book recently and told me that she had four separate email addresses for me. My response was "Only four?".
When you have multiple accounts configured in Outlook, the account that's used to send mail varies based on several things:
I did that later step repeatedly. In test email after test email, I explicitly told Outlook to use a specific account. Outlook apparently ignored my request, and used a different account anyway.
As it turns out Outlook will, under certain conditions, select a different outgoing email account if it has trouble sending on the designated account. Of course it does this silently.
My solution was to visit the configuration for the account I was having trouble with, and send a test email. It asked me for a password, which I knew it shouldn't have. In my case, the sending authorization configuration for this account had been mistakenly changed to "Log on to incoming mail server before sending mail". That's not how my mail server is configured. Changing the configuration to "Use the same settings as my incoming mail server" resolved the issue.
Needless to say it was not obvious, and very frustrating.
So the lesson here is simple: if Outlook suddenly starts sending using the wrong account, check your ability to send via the right account. There might be a problem there.