It is not possible to send mail from specific domain, using Roundcube webmail. The following message is recorded into
plesk sendmail: handlers_stderr: DATA REPLY:554:5.7.0 The message could not be sent. You are not allowed to use sendmail utility.
plesk sendmail: handlers_stderr: REJECT
plesk sendmail: REJECT during call 'limit-out' handler
postfix-local: Unable to forward original message. sendmail status: 100
The domain name contains upper-case characters.
The issue is recognized as a product bug with ID PPPM-1941 , fixed since Parallels Plesk 12.0.18 MU#33 .
Install all Plesk Microupdates in order to have the issue resolved.