Somehow, my postfix configuration got into a state where new email users can receive but not send email. However, if I add their IP address to Webmin->Servers->Postfix->General Options->Local Networks, they can send.
For users that have valid SMTP credentials it doesn't make sense to me that they would be rejected based on their IP address. What if they travel and login via a hotel's wifi network?
Note that the emails that are failing have a different FQDN,
abc.com than that which appears in
Webmin->Servers->Postfix->General Options->Local internet domain name: xyz.com. Moreover, the FQDN
Webmin->Servers->Postfix->General Options->Internet hostname of this mail system: mail.xyz.com is, again, different from either
PS: I've always found confusing the configuration of postfix to support multiple virtual server domains. It doesn't make sense that one should have to choose only one of the many virtual server domains for the aforementioned fields. Intuitively, postfix should resolve based on the FQDN of the authenticated email address.