SMTP-Error 554 if domain has capital letters

[Alexander79]

Hello,

my Plesk-Version is:12.0.18 Update #13

After the update some of the Clients gets following error-message while sending Mails:
SMTP Fehler: [554] mail server permanently rejected message (#5.3.0)

The Problem accours, if the domain name is written with first letter in capital letter:
Problem: Domain.com
No Problem: domain.com

If I delete the Domain and add it new without a capital letter, there is no problem.

Problem is reproducible.

Regards, Alex