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In short, I'm running Postfix and Dovecot on a server and everything is working. The issue is that when sending mail with the firewall up, Thunderbird hangs for about 10 seconds before sending a message. The message makes it to the recipient as expected but the delay is consistent on the sending side. Receiving mail seems to take longer as well.

I noticed that if I disabled the firewall the problem goes away entirely. Does this sound familiar to anyone? Is there a port I need to open?

Mureinik
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user2864874
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  • Have you tried looking at firewall logs to see if it has blocked something? Or a packet capture to see if Thunderbird is sending packets that remain unanswered? – u1686_grawity Oct 10 '21 at 07:01

1 Answers1

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This sounds like either a) a lot of packet loss b) slow hardware or c) slow email relay. If you are using IPtables, this script is all you need for a very basic outgoing SMTP firewall, but it is not a full firewall and will not work for any other service except SMTP. It will log packet drops in case you have any. Make sure you are using the standard email ports (25,465,587).

$IP='/usr/sbin/iptables'
$IP -t filter -A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
$IP -t filter -A INPUT -p tcp --dport 25 -j ACCEPT
$IP -t filter -A INPUT -p tcp --dport 465 -j ACCEPT
$IP -t filter -A INPUT -p tcp --dport 587 -j ACCEPT
$IP -t filter -A INPUT -j LOG --log-prefix "INPUT DROP: "
$IP -t filter -A INPUT -j DROP
$IP -t filter -P INPUT DROP
Cameron
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