Mac Forum / Applications / Eudora / May 2006

I don't think so, at least not automatically. Ordinarily Eudora uses the
personality associated with the incoming message when creating replies,
but you can use the drop-down list to change that.  For new messages,
the dominant personality is used by default, but again that can be
changed on a per-message basis using the drop-down list.

I don't believe Eudora will use a From address that isn't among the
defined personalities; you need to have a server specified for it to be
able to attempt to send the message, and that is done in your setup.  

FWIW, I have an address kmorgan@myisp.example (made up to protect from
spam, but analogous to the real address). I have some old messages that
I received addressed to <kmorgan+nai@myisp.example> from when my ISP
correctly delivered mail addressed to username+whatever.  When I attempt
a reply to those messages, Eudora uses the personality associated with
<kmorgan@myisp.example>.  Eudora apparently ignores the portion of the
username following the "+".

The only time I can think of where this would be an issue is if you own
all the addresses for a domain and have some sort of form that allows
people to send you mail to <some_new_username@domain.example>.  Since
you would be getting all mail sent to anyone at domain.example, you
would want to be able to reply but have the From address be one of the
personalities you have defined.  I think Eudora would use the dominant
personality, or maybe the first personality associated with
domain.example, but that's just a guess.

If instead it tries to create a reply using
<some_new_username@domain.example> which doesn't have server informtion
defined, I believe it wouldn't be able to actually send the message, and
you'll get an error response with a bright red stop sign to alert you to
select a valid personality.

If this is your situation (you own domain.example and receive mail from
addresses made up on the fly), I'd say to do some experimenting and see
what happens.  send some messages from a valid personality addressed to
for instance test1@, test2@, and see what happens.  And if you do that,
please report back with the results.  Inquiring minds want to know. :-)