"Configure Email Client" windows display wrong "Incoming mail server" and "Outdoing mail server" - it displays domain name instead of full mail server name like in MX record.
MX record has got non-domain value in Plesk > Subscriptions > example.com > Websites & Domains > DNS Settings
while in Plesk > Subscriptions > example.com > Mail > Email Addresses > > Configure Email Client
Incoming mail server and Outgoing mail server parameters have got the domain name values:
It is known issue PPPM-4687 which will be fixed in future updates.
All information regarding Mail server username, Incoming mail server, Outgoing mail server parameters and etc. can be found in the file
As a workaround, create a backup of this file and edit it according to your needs.
For example, editing of this row
<?php echo $this->lmsg('fieldIncomingServer'); ?>: <?php echo $this->escape($this->domainName); ?><br>
to this one
<?php echo $this->lmsg('fieldIncomingServer'); ?>: <?php echo $this->escape(12345); ?><br>
changes Incoming mail server to 12345.