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frame 1 [syn] -> frame 2 [rst, ack] on port 25 of remote server

Hi

My postfix smtp server suddenly stopped working for no apparent reason. I now find that I am getting a [rst, ack] packet back after the initial [syn] packet.

I'm trying to connect to port 25 on the remote server. This affects all outbound mail. I've checked all postfix configs, all inbound and local email works, dns and rdns all configured correctly, not on any blacklists and I don't relay through another server. I can understand one server not wanting to respond but I'm completely baffled. I can't see that all mail servers would deny access on port 25.