On 6/1/2020 10:29 AM, Jerry Geis wrote: > The problem is the client program trying to talk to the linux server > > is base64 encoding the entire email address for the AUTH LOGIN, Note that it's not talking to "the linux server" but to a specific program on the server.I'm guessing Postfix. (But maybe Sendmail, depending on how old your server is.) So the question is whether Postfix or Sendmail have this ability to translate an incoming username. They may be able to do this through virtual user support. Which SMTP program are you using (name and version)? I'd suggest asking on their mailing list.