Issue Mail settings fail to save correctly

[sjerpbouwtsites.nl]

Environment:

OS: CentOS Linux 7.9.2009 (Core)
Product: Plesk Obsidian 18.0.35 Update #2, last updated on June 1, 2021 07:29 PM

Behaviour:
1. open email account settings, in this case 'info@vloerwerk.org'
2. notice odd IT-complaints@vloerwerk.org mailaddress.
3. try to change 'back' to info@vloerwerk.org
4. IT-complaints returns after save.

I'm having some other issues with mail, might be related, might not be.
All other emailadresses give the same behaviour.

I've tried the various repair functions but no luck.... anyone?