Well, @mantas8 and I worked on this and the issue is now resolved.
I had to do the following to resolve the issue:
- create a subdomain, vps.domain.tld, for hostname of server Virtualmin was running on
- change the hostname from the default that Contabo sets to vps.domain.tld
- create subdomains for name servers
- Rerun Post-Install Wizard and apply name servers
- Request SSL certificate for vps.domain.tld and apply to webmin / usermin / postfix / dovecot et al.
- Install Roundcube
- Troubleshoot SASL (as Joe had recommended in the earlier message)
- Send / receive test messages from Roundcube and Thunderbird - everything worked perfectly right off the bat
Thanks for the business @mantas8!
Hey @saabir, you had a similar issue too. Hope this sequence of steps helps you fix it.