Postfix/smtp Connection timed out even though ISP not blocking port 25

postfix

I just configured postfix/dovecot on a VPS. I can receive but I cannot send emails to any destination except to gmail which shows my email as a spam. Below are the symptoms:
**Symptom1:** Emails sent to non-gmail addresses:
**example1:**
*Dec 29 07:51:39 tingene postfix/smtp[9815]: connect to smtpin.zoho.com[136.143.191.23]:25: Connection timed out
Dec 29 07:51:39 tingene postfix/smtp[9813]: connect to smtpin.zoho.com[136.143.191.23]:25: Connection timed out*

example2:
Dec 29 07:51:39 tingene postfix/smtp[9814]: connect to us2.mx2.mailhostbox.com[208.91.199.202]:25: Connection timed out

Symptom2: Emails sent to gmail addresses and received as spams in gmail:
Dec 29 07:51:53 tingene postfix/smtps/smtpd[9816]: timeout after END-OF-MESSAGE from xxxxxxxxx[xxx.xxx.xxx.xxx]
Dec 29 07:51:53 tingene postfix/smtp[9821]: connect to gmail-smtp-in.l.google.com[74.125.137.26]:25: Connection timed out
Dec 29 07:51:55 tingene postfix/smtp[9821]: 848A52111D: to=myemail@gmail.com, relay=gmail-smtp-in.l.google.com[2607:f8b0:4023:c03::1b]:25, delay=32, delays=0.42/0.02/30/0.81, dsn=2.0.0, status=sent (250 2.0.0 OK 1609228315 d17si42881479pgg.14 – gsmtp)

My ISP is not blocking port 25. This is the proof:
root@tingene:/var/log# telnet gmail-smtp-in.l.google.com 25
Trying 2607:f8b0:4023:c03::1b…
Connected to gmail-smtp-in.l.google.com.
Escape character is '^]'.
220 mx.google.com ESMTP v10si41545875plz.191 – gsmtp
^]
Any advice is appreciated.

Best Answer

Your ISP is not blocking port 25/TCP on IPV6 connections but seems to block this port on IPV4 connections:

  • the test you did with telnet used IPV6.
  • The logfile snippets you posted show that your server tried to connect to the google smtp server two times. first time via IPV4 (failed), second time via IPV6 (succeeded).
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