This is not easy to fix.  Each way has potential draw backs :(

Workaround - reload the page

Might get to it one day

Section of IRC discussion for further clarification of problem:
<bjohnson> I've now tried both a user to a non-fqdn and a non-existent user to our fdqn
<bjohnson> both resulted in error screen from postfix and nothing saved in sent folder (we use postfix for mta/smtp and courier for imap on a Redhat 9 machine)
<bjohnson> after the hitting send on the compose screen I get a screen with an error message from postfix
<bjohnson> hitting browser back button I get an empty compose email screen
<bjohnson> email message must be saved in memory to send and then move to sent folder
<bjohnson> maybe just change in code order to put in sent folder BEFORE sending to smtp server

When creating an e-mail in poopware yesterday, there was an
error in the sender's address, so the e-mail did not go. When I
went to find the "draft" of the e-mail, it was nowhere to be
found (like you can in Outlook) to resend it again with the
correct e-mail address. I looked to see if there was a place
for "unsent" messages to be set up or created and I did not see
any options for that type of thing.

Are you out of luck when you have typed in an incorrect e-mail
address?

Angela

I checked into this and can confirm it happens

Could emails be saved in the Sent folder (if selected in user prefs) BEFORE it contacts the smtp server .. this slight change in program order would bypass smtp errors and a saved copy would remain