I have two domains configured, call them group.example.com and example.com. The email address I'm trying to authenticate with is
user@example.com, but I think it's trying to authenticate as
user@group.example.com. I had to put in the mail server as group.example.com in Thunderbird for DNS purposes since my current MX record for example.com points to a different live mail server. I changed the account so it authenticates as
admin@group.example.com and it successfully logged in. Consider this problem resolved.