Question Plesk Onyx 17.5 - SMTP response to slow

[Haecki]

For some days I noticed that the SMTP connection to my server takes very long time to respond to the client (Outlook, Thunderbirt).

Whether a secure or unsecured connection, I ask the server by telnet, I received for some time only after 10 seconds a "welcome" message.

This annoys extremely, since some IP cameras now no mails can send, since these already after 5 seconds the connection again abort.

I thought it was possibly a firewall problem, but I have intensively examined this and can also see in the network monitor immediately the request for sending from my IP address, it seems here the postfix something to brake.

Before rolling out the last update from 14.06. It had worked temporarily
But since yesterday the SMTP is again very slow.

What can be the reason, how can I analyze an error here more?

 

[AYamshanov]

Hi,

Check DNS from system network settings, it should answer to requests.

 

[Haecki]

What exactly should I check? The DNS server is assigned in the configuration via DHCP from the data center of the network card.

 

[AYamshanov]

It is only my opinion: the DNS server could resolve IP-address of client to domain for adding it to log-files. And mail server waits for the answer from DNS or timeout.

Or mail server trying check ip-address of a client via blacklists (unavailable blacklist?)

 

[Haecki]

OK thanks, the DNS server solves everything correctly, I tried it with Telnet with the IP of the e-mail server and my IP address is server-wide in iptable and other protective measures "white". The ports are all open - to test!
In the logfiles is only that the connection from the client was successful but before the connection was terminated.