If I have a home mail server, what could prevent someone from sending me an email at my IP address?

Question

I've found that my 2 domain providers both prevent me from putting my static IP address in the MX record for my domains.

But let's say my IP is 71.222.111.33.

If I open up port 25 on my Wifi router and have a mail server listening for incoming connections, what would prevent someone from successfully sending an email to warren@71.222.111.33 ?

Answer 1

Have you asked your ISP if they permit home e-mail or other servers? In many cases the ISP will simply block any traffic to certain ports like 25. As the traffic passes their router(s), it is either dropped or blocked in some way. Same for HTTP and HTTPS and many other protocols. If you by a "business" or "commercial" package from them, then the traffic passes without issue. But you are now paying a much higher price.

Answer 2

As explained in section 4.1.3 of the STMP specification (PFC5321#4.1.3), you have to use an address literal if the host has no name. The correct syntax would be:

warren@[71.222.111.33]

As by the standard, this is supposed to work. I tested it, and it works fine with Thunderbird and Postfix.

But there are email clients and/or mail servers that will choke on this syntax (basically because they don't respect the standard). For example, the email provider WEB.DE marks that address as invalid.

Answer 3

This is not a direct answer to your question, however it may be a solution to your problem.

If your domain registrar allows you to change the Nameserver records for your domains (which all good registrars should) you can use a 3rd party DNS management provider to set your static IP as an MX record.

I have used ZoneEdit for many years.

Answer 4

There are a few reasons your email might not reach it's destination.

ISP Restrictions

Your Internet Service Provider might not like you having your own email server, by blocking all email servers they significantly reduce spam on the internet and email remains a reasonably trustworthy communication medium.
This is most likely done by blocking common email ports (25, 110, 143, 465, 993, 995, 2525).

Email Provider Restrictions

The Email Provider might not like untrustworthy IP addresses as destination for outgoing emails, thus does not allow numeric addresses as destination, only domain names (sometimes they only allow a list of trustworthy email servers).

Routing Issues

The Email Server that sends the email might be unable to find the correct route to your server for some reason, this is a bit unlikely if you use your unique, external IP as server address.

Misconfiguration

You might have misconfigured some things, like port forwarding incorrectly configured or a non-functioning email server.

IP Address Renewal

Your ISP most likely gave you a dynamic IP address, when your lease expires you might be assigned a different IP then you sent the email to, this can be solved by using a dynamic DNS service such as No-IP.com that uses a client to update your dynamic DNS record to your current IP whenever you obtain a new IP.

Power Outages and Similar Events

Of course there can be special circumstances that render your email server inoperable. But this is highly unlikely.

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I'm home-hosting a Raspberry Pi with some services like a VPN, SSH, and HTTP. But no email since i discovered my ISP blocks port 25.
I'm using noIP for dynamic DNS services, although i don't really need it because my ISP has not changed my IP for over a year.