Mike Trout wrote:

They are on different domains. They are also separate on the hosting
server. I go into webmail for thisdomain and log in using the account
(name) and the password, and it sees only thisdomain messages. I go
to webmail for thatdomain and logon using the account (name) and the
password (which is different from the password for thisdomain, and
per our hosting company the account + password defines a mailbox),
and I see only thatdomain messagses.

As I said, the domain means nothing if all of the domains are actually
hosted by the same company. What you describe after, though, seems to
indicate that even if the mailboxes are on the same host, they are distinct,
so Outlook shouldn't get them confused.

Thank you for sticking with me on this, Brian. I personally think
the user should just check the account before clicking the send
button, but it shouldn't do this in any case, and she's complaining
to my boss, so...

One thing they might try is to create separate send/receive groups for the
accounts.
--
Brian Tillman