Mail subjects in Plesk at Tools & Settings > Mail Server Settings > Mail Queue tab have /var/spool/postfix/deferred /X/XXXXXXXXXX.
Mail subject is 100 and more characters.
This is a Plesk bug with ID #PPPM-9533 which is planned to be fixed in one of the future product updates.
This issue affects only the view in Plesk User Interface, mail headers stay untouched so the issue can be safely ignored.
As a workaround use mail subjects less then 100 characters.