Many local ISPs started blocking port 25 recently. My clients can no longer access their email accounts on their email server from external clients like Outlook, etc.

I am a complete rookie in this area and I read that port 587 would work as a replacement port. Before I change the port in the server I would like to know if this is a standard widely used port and if anyone has any stories about emails not arriving at an email server whose SMTP port is 587.

I would also like to know if there are any alternatives to changing the port.

  • 1
    I think your question needs some additional information for clarity. Which direction are we talking about here - inbound from the internet towards the email server or outbound from the LAN towards the internet? Additionally, are the clients you are referring to on the LAN or remote trying to retrieve email?
    – Tall Jeff
    May 30, 2009 at 18:23

5 Answers 5


587 is the mail submission port. it lets your clients post mail which is then delivered to other servers via smtp (port 25 on destination system) incoming mail from none-clients for your domain will allso be on port 25 on your system

  • Thanks! I just uncommented the following "submission inet n - n - - smtpd" in the master.cf of my postfix config and now I can send emails from external clients through the port 587 without having to touch port 25.
    – user6271
    May 30, 2009 at 19:43

We have used port 26 as an alternate SMTP port for years now, without conflict. This is useful when client's ISPs block port 25, preventing you from sending outgoing mail through another server.

There are two methods of doing this, one is to forward traffic from port 25 to port 26 via your firewall. Or the recommended method, of setting your MTA to listen to port 26 also. Either way you will need to allow your firewall to permit traffic through port 26.


The problem you described seems to me on Outlook from external client where they can't send email out. If so, you may adjust the smtp settings on their outlook to use smtp server provided by local ISPs. It's a common practice that ISPs blocks port 25 from other sources than their own, as it can lead to be spam source very easily. You do not need to change the port on your email server.

Cheers, Kent


Port 587/tcp is often used for SMTP+TLS. Gmail uses it, for example.

But whatever you do, you should not disable port 25.


You wont be able to receive mail to an alternate port, smtp doesnt work that way. So if your objective is to be the MX for a domain, you're out of luck.

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