Cannot outbound port 25 - others are working

Hello Forum,

I just set up my first Proxmox environment with some VMs and CT. Working good so far.

Now I want to reach out my external mail system to forward internal mails to external. But connectiong to external mail system does not work with port 25. I can reach out every other port like 587 or 465, but not port 25.
My ISP is not blocking port 25 because I have another client in my internal network where it is working.
I disabled all firewall in Proxmox.

I do not know why exactly this...

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