Many thanks for your help.
We have added SRV records to external domain provider and unlocked VPS mail ports and looks like everything started to work.
Hope that helps someone in the future.
Hello,
We have recently moved to a plesk managed hosting with ionos and have serious problem with emails.
our domain: badkamermateriaal.nl, hosted with rootnet (outside of ionos)
our hosting: ionos Cloud Server XL with plesk
OS Debian 10.9
Product Plesk Obsidian
Version 18.0.35 Update #1...