As per notification emails sent during December and January, legacy SMTP users (i.e. those that use the domain name only and not the FULL email address as the user) are now disabled.
Users who are able to receive mail fine but are suddenly not able to send should check the user name and password they are sending with and select the option "use same settings as incoming server".
Resellers on our OLD platform (these are the ones using Plesk 8.x) should additionally check the DNS records. of any domain that experiences issues. Any smtp record pointing to smtp.xenserve.com should be changed to an A record that points to the IP address of the reseller server itself (you can confirm this by looking at the other A records for the domain).