It may be blocked by the ISPs that service a majority of people (i.e. cable companies), but I don't have any reason to think that it's blocked by a majority of ISPs.

Indeed. Chances of a random person accidentally setting up an open email relay of some sort are high.

Although an open email relay would probably be some other violation of the AUP, methinks.

(warning: haven't actually read AUP.)

I actually butted heads with this part of the AUP when I was a teenager.

I was young, naive, and trying to set up my own mail server. Ended up creating an open SMTP relay instead.

Received a phone call within 15 minutes of successfully testing my mail server! They were calling to tell me they suspect a computer in my house has a virus. I chatted with support for a while and we eventually figured out it was my mail server.

They were rather pleasant about it actually, they explained that it doesn't necessarily violate their AUP but it's hard to configure a mail server correctly.

I'd later learn that since my IP address was from their dynamically allocated pool: most popular mail servers would reject my messages as spam anyways.

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This was with TimeWarner cable about 7 or 8 years ago.

Most likely. But it's trivial to see whether or not someone is running an SMTP server whereas it's more difficult to determine if they're running an open relay.

Wouldn't it be a simple matter of just trying to send an email using the server in question? If you receive the email you just sent, then it's an open relay.

So now you have to have an email account somewhere that allows anything and everything to be sent to it (no spam filtering), and you need a system to send emails and correlate that with received emails. This is all certainly possible and not necessarily that difficult, but compared to seeing if a service is running on port 25 it's about a hojillion times more hassle.

You don't need to send. Send "helo", "mail from" and "rcpt to" commands and most servers that are not open relays will reject the message after the rcpt to. Issue a "data", and most remaining ones will. At which point you can disconnect without sending any e-mails. Open relay testers are dime a dozen.

Surely yes, but they probably don't want to go through the trouble of tracking them down one by one- and doing so on a continuing, never-ending basis.

I'm surprised to hear that. I've never had a problem with an ISP blocking SMTP.

Inbound on port 25? I know my ISP does

Mine doesn't (Roadrunner). I run my own SMTP server off my cable modem. I have no problems getting incoming mail. Outgoing, however, was a pain because so many SMTP servers block mail coming from dynamic IPs. It annoys me to no end. I understand the reasons for doing it, but it annoys me to no end. I had to set up a small satellite server on a static IP that I route all my outgoing mail through.

This leads me to wonder why you run any portion of your mail server from your home if you have access to a small satellite server?

To search your home, a warrant is needed. To search a remote site, less is needed, and you might never know it’s been searched, back-doored and compromised.

Mostly because the small satellite server requires very little storage, cpu, or memory. I have about 100G of mail on my IMAP server (not all in one account: I run email for everyone in my family), and that kind of space isn't cheap in the server rental business (despite the fact that a 2TB drive is $90).

Plus I just like having my data here, and not somewhere where I have no physical access.

Come to think of it, I've never looked at inbound stuff. You'd think outbound would be where all the spam problems come from, though.

> You'd think outbound would be where all the spam problems come from, though.

They block port 25 to prevent open relays on their network (i.e. someone sends a forged email, the SMTP server does no authentication, and just forwards it on to the destination).

If you block 25 outbound, people's mail clients (Outlook, Thunderbird, etc) could not deliver mail to servers that don't run SSL or TLS (465, 587)

Lots of ISPs does this. They require you to relay via their SMTP-server.

Ideally you would use port 587, which is the designated port for mail submission. See RFC 2476 [1].

This requires that your mail provider supports that port, of course, which is unfortunately not always the case.

[1] http://www.ietf.org/rfc/rfc2476.txt

Used to have Bellsouth DSL, and they did this. SMTP/TLS on a different port worked though. But obviously you couldn't run your own e-mail server.