My unmanaged VPS server's host company sent me a new warning about a significant increase of outgoing connections from my server: The level of SMTP traffic on port 25/tcp is unusually high and at this pace you will hit the limit soon and all connections on this port will be blocked until the next day.
I have 7 sites hosted, used to have 11 email accounts on some of them and blocked the email server altogether a couple of months ago when I received similar warnings as I couldn't identify the issue after blocking certain IP addresses which appeared to be repeatedly trying to access port 25. I stopped receiving the warnings since then until 2 days ago, and it's back again.
Port 25 is blocked, what am I missing?