I have a mail server with postfix and dovecot. The mail server is on a VPS. At the moment, I can send/receive mails from the server itself. It seems to work OK.

However, I'd like to use my mail addresses from my laptop. I configured my account, set up the IMAP and SMTP. IMAP works, I can receive my mails on my laptop.

But I can't send any mail.

I looked up the traffic using tcpdump, I could see SMTP packets going out of my laptop, but nothing comes at my VPS.

After a few research, it seems that my ISP is blocking that traffic. What am I supposed to do? It seems strange, because we can use SMTP to send mails with gmail and other mail services.

I used the online tool mxtoolbox and everything seems ok

|      | Test                      | Result                                   |
| Warn | SMTP Reverse DNS Mismatch | Reverse DNS does not contain the hostname|
| Warn | SMTP Banner Check         | Reverse DNS does not match SMTP Banner   |
|  OK  | SMTP Valid Hostname       | OK - Reverse DNS is a valid Hostname     |
|  OK  | SMTP TLS                  | OK - Supports TLS.                       |
|  OK  | SMTP Connection Time      | 1.000 seconds - Good on Connection time  |
|  OK  | SMTP Open Relay           | OK - Not an open relay.                  |
|  OK  | SMTP Transaction Time     | 3.047 seconds - Good on Transaction Time |

Connecting to W.X.Y.Z

220 example.com ESMTP Postfix [641 ms]
EHLO PWS3.mxtoolbox.com
250-SIZE 10240000
250 DSN [656 ms]
MAIL FROM:<supertool@mxtoolbox.com>
250 2.1.0 Ok [656 ms]
RCPT TO:<test@example.com>
451 4.3.0 <test@example.com>: Temporary lookup failure [656 ms]

PWS3v2 7391ms
  • 4
    Are you using port 25? If so, that's likely the problem. You should be using the SMTP submission port, 587/tcp. – EEAA Sep 10 '15 at 16:05
  • Have you configured postfix to allow your laptop to use your server as a mail relay? By default for any external incoming connection, postfix would only accept mails whose recipients belong to its domain. If you want to use postfix from an external client (like your laptop) to send mails to an external domain, you need to edit the default config in postfix to allow authenticated users, fixed IP origins, etc – NuTTyX Sep 10 '15 at 16:28
  • Any logs from /var/log/mail.log? What's your configuration? – sebix Sep 10 '15 at 18:15

From your laptop you want to use the submission port, TCP/587, port 25 is dedicated for server to server routing via MX records.

| improve this answer | |

Laptop? Seems to me you are trying to connect from out site your network. What about the firewall?

I use to delete all "Trusted Networks" from SMTP Config, so all SMTP Clients have to authenticate thru the server. Do you have your SMTP Client configured this way?

Postfix have worked for me on port SSL 465. Open that port in firewall for incomming connections.

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