On 6/1/20 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, > not just the "username". so my user name needs to include the "@" symbol. That's a common requirement for servers that implement multi-domain hosting. In Courier MTA, you can host virtual users in a "userdb" file or SQL DB (or LDAP): https://www.courier-mta.org/FAQ.html#virtual Postfix supports similar setup: https://www.howtoforge.com/virtual-users-and-domains-with-postfix-courier-mysql-and-squirrelmail-debian-wheezy Whether those are options may depend on how invested you are in sendmail. If you're not a sendmail expert, you're probably better off using a different mail server.