Could not deliver message to the following recipient(s):

Failed Recipient: <anyone>@gmail.com
Reason: Remote host said: 550 5.7.1 our Bulk Email Senders Guidelines. 50si6194057ugb

What does this mean? In a nutshell, GMail server thinks I am a spammer. Unfortunately, there are no links leading to more information in the text that follows and there are no hints as to why does it think I am a spammer.

Searching the ‘net will lead you to the mostly useless GMail Bulk Email Senders guidelines. I say “useless” because the text applies to those sending a lot of email to GMail servers, but also because I wrote them a complaint and got back what looks like an automated, generic reply (please make sure you’re not sending a lot of mail blah, blah). I send at most about 5 (five!) emails a day to GMail, so this can’t be the problem.

Then I remembered how email server at work used to reject emails from the account I was given by my ISP. I also remembered how due to the high amount of spam sent from hijacked computers on the network of my ISP, recently that same ISP started blocking port 25 by default for all consumers.

Since I run my own email server I had to explicitly opt out of this, but it made me think – could it be that an over-zealous third party put a large range of IP addresses from  my ISP as potentially spammy (due to bad history) and my IP address was accidentally included? In order to fight spam, there are several organizations, some commercial and some not, who maintain a list of bad (and sometimes also a list of good) hosts/IP addresses. If you are on a bad list, you are blacklisted.

You can check if you are blacklisted here (click on the first link called rbls). I was a bit surprised but my IP was listed, fortunately just on two places. I am guessing here but I think GMail is using the services of the one I heard about before called Spamhaus (I was listed there). There is a link leading to each and every blacklist site where you are given a chance to remove yourself. Sometimes (this was the case with Spamhaus) an email confirmation will be sent to your email address as a verification step but sometimes a single click is enough. Note that this does not mean you’ll be whitelisted forever – if you are a spammer and sending a lot of email, you’ll get blacklisted again.

In my case, an hour or so later everything went back to normal (it takes time till the removal of your IP address propagates through the ‘net).