Exchange 2016

This error related to two main factors

Remote Server returned 554. One belongs to retention policy. Normally exchange administrator change a retention policy from 14 days to 60 days. mailbox size when get larger than 20GB attachment bounce back. Some company setup exchange 2016 attachment from 10MB to 35MB or 45MB. workaround this issue change a retention from 60 days to 14 days. increase mailbox quota size.

Second problem will be mailbox delegation issue. remove all users from mailbox delegation setting and problem solved.

Remote Server returned 554

for details information please follow this Microsoft link

Exchange Server Specialist.