learning_curve
Golden Pleskian
The small extract that you've posted, shows that you're using IONOS MX settings for your e-mail... Plus you've posted two different IP addresses...On my new Plesk Server:
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In my Maillog:
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My hosting provider has enabled port 25 outbound. I also talked to him on the phone, he can't find any error.
I have not installed any firewall extension in Plesk. What else can it be?
Did you want (and therefore, have you actually properly configured...) mail service, on the domain that you're testing on, to be hosted & configured in Plesk?
If yes, then you'll need to re-examine your IONOS settings re: MX records first of all... (we use IONOS but cloud servers not VPS, but it's the same principle)
No idea of the domain name that you're using to test, but both of those IP addresses thatc are in your post, report some failures when tested on Mail To > https://www.checktls.com/TestReceiver Only you can test the Mail From > https://www.checktls.com/TestSender but these (and all other test sites) can be used for reference, once you've resolved your IONOS / Plesk interface configs, which, appears (from what you've posted so far) to be your starting point...