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I am trying to set-up Mikrotik (RouterOS v6.24) for sending emails.

I have google account and I am using SMTP connection. I know that I need to allow outbound connection for it to work. But when I try to send an email, the inbound filter will block connection and sending an email will fail.

16:37:04 firewall,info input: in:ether1-WAN out:(none), src-mac 00:13:60:16:4f:c6, proto TCP (SYN,ACK), 74.125.128.108:587->x.x.x.x:5462

When I disable the input filter. The email will be send correctly.

Why do I need to allow input connection for sending an email?

RouterOS settings:

   address: 74.125.128.108
       port: 587
  start-tls: yes
       from: xxxxx@gmail.com
       user: xxxxx
   password: xxxxxxxx
last-status: failed

Command used:

send to=xxxxx@xxxx.com from="xxxx@gmail.com" subject="test email" body="test body"
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It appears that you don't allow established/related connections on the Input chain.

In order for the router to communicate with the outside world (be it smtp, or anything else) and you have a firewall on the input chain you need to allow any established/related connections back in to the router.

Simply add on the top of your input chain the following rule:

/ip firewall filter add chain=input connection-state=established,related action=accept
  • Thanks, this fixes the issue (or adding allow for google server). But could you please explain, you mentioned (or anything else), even when we did not have this rule in our firewall and we simply blocked all incoming traffic - we could use the internet on the devices behind the firewall without any problem. I would imagine that without that rule it should not work. – pagep Sep 14 '16 at 11:53
  • Without allowing established/related connections and without explicitly allowing the remote IPs your router tries to connect to, then the replies back from those remote IPs will be dropped. TCP/IP is a bidirectional protocol. You send and receive packets, so the router's firewall needs to know which of the reply-packets to accept (ie: established/related) and which to drop. That's where the 'established/related' rule comes into play. For more information on the subject you may lookup 'Stateful Firewall' and iptables. – Cha0s Sep 14 '16 at 13:16

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