How to open port 26

Hello,

my ISP is blocking port 25, so I would like to try port 26 to handle mail.

How can I open port 26 in ip tables?

Thanks

if you do not use firewall you do not need to change iptables, you need to change server to listen on port 26.
if you are behind some router you can do it in router by forwarding external port which you want to internal 25...

here you have tutorial for postfix: http://sharadchhetri.com/2013/03/06/how-to-change-smtp-port-number-25-in-postfix/

At the roundcube webmail I try to send a test mail and get:
SMTP Error (-1): Connection to server failed.

Renamed SMTP to 26, in terminal I get
tcp        0      0 0.0.0.0:26                  0.0.0.0:*                   LISTEN      16129/master
and some other

So, Connection to server failed. code -1 does that mean autentification error???

Did you restart mail server after change?

netstat -nlp | grep :26

check is listening on 26 port...

Yes, restarted the postfix mail service.

Get this:
tcp        0      0 0.0.0.0:26                  0.0.0.0:*                   LISTEN      22161/master

Firewall is enabled, maybe it is connected somehow?

seems ok to me try to telnet remotely on that port and see is there response or scan with some tool to see is it opened, if its opened should work but to be shore check email log and see is there some errors.

Port 26 is open.
Mail log have errors, mails are in queue:

host postfix/qmgr[2803]: 8FF0F240A35: to=<dell.webserver@gmail.com>, orig_to=<root>, relay=none, delay=77, delays=0.03/77/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Jul 13 14:45:41 host postfix/smtp[19518]: connect to alt2.gmail-smtp-in.l.google.com[173.194.72.26]:25: Connection timed out
Jul 13 14:45:41 host postfix/smtp[19518]: connect to alt3.gmail-smtp-in.l.google.com[74.125.25.26]:25: No route to host
Jul 13 14:45:41 host postfix/smtp[19524]: connect to alt3.gmail-smtp-in.l.google.com[74.125.25.26]:25: Connection timed out
Jul 13 14:45:44 host postfix/smtp[19518]: connect to alt4.gmail-smtp-in.l.google.com[74.125.198.26]:25: No route to host
Jul 13 14:45:44 host postfix/smtp[19518]: 675082408ED: to=<dell.webserver@gmail.com>, orig_to=<root>, relay=none, delay=93, delays=0.09/0.06/93/0, dsn=4.4.1, status=deferred (connect to alt4.gmail-smtp-in.l.google.com[74.125.198.26]:25: No route to host)

Any ideas?
Router is blocking something?
Ports are opened!

check the router first, logs show you cant connect to gmail server something is blocking if router doesn't have any restrictions there maybe its on provider side...

What do I need to check at the router?

You need port 25 open, if cant,  useless
Another way is relay to another server,  will work with other port than 25

Will ask at Mikrotik forum how to do that! Thanks for the help guys 

One more thing, can I use google smtp ip 173.194.77.108 as a relay?

yes you can use google smtp port 587, at very limited daily rate.
afaik, outgoing email address will (forced) change to use authenticated user for smtp submission.. i hope you understand what i mean 

these links may usefull for you
http://www.jscape.com/blog/smtp-ports
http://blog.simplehost.web.id/2016/02/23/set-up-postfix-for-relaying-emails-through-another-mailserver/