Email providers use automated systems to determine most blocks, and these systems do a number of tests to determine who should be blocked, and for how long.
Email Volume - if your server regularly sends out LOTS of mail to the same destination network from the same IP address within a short period of time, you may end up having a portion of your mail “deferred” or even “blocked” as the provider may see the sheer volume as an attempt to SPAM their network. In many cases this is a false positive, and you need to reach out to the provider by whatever means they’ve provided and explain this. Sometimes you can auto remove yourself temporarily via an automated method, other times you need to wait til a human reads your message and addresses the matter.
SPAM Content - if you send spammy content to a provider’s network, and by spammy I mean by their definition, then you may see yourself getting blocked for either a short period (to deter you from continuing) or long term (requiring contact with them). Sometimes the provider’s spam filtering system is a bit anal and too protective, other times your messages themselves while seemingly harmless come across as being spammy.
User Complaints - most large providers these days offer a “This Is Spam” button within their web interface. If too many people click this in connection with your IP address or even sometimes a neighboring IP address in the same netblock, you may end up getting blocked. As always, it is your responsibility to reach out and ask the provider why they blocked you (if it’s not obvious) and work with them to have the block removed.
IP Address - if the IP address your mail server is using is fairly new, or hasn’t been used with email for a while, you may need to “warm up” the IP address by sending small volumes of mail to a provider over a period of time in order for them to build trust with you. This is annoying, but often necessary.
There are lots of other reasons for being blocked, and like I said initially there is no “proper” way to get around being blocked these days. Each provider sets their own rules and guidelines so if you stay off one networks blacklist, it doesn’t mean you’ll stay off the next ones.
I am the system administrator of a large email network which sends millions of messages daily. I’ve run into many blocks over the years, and have had to come up with some creative ways to address them, and even threaten legal action against a few providers who were outright uncooperative or dragging their feet on a resolution.
Even still, issues present themselves from time to time, and it often feels like a full time job keeping the blocks at bay.
Thank you for jumping in. It is not the miraculous solution I was dreaming of … but I am fully aware it was just a dream and this is still a very nicely crafted answer
I still struggle to understand why I experience troubles with Hotmail only and why an IP (that was clean in the first place) ends up being blocked by them after a few months while having sent no mail during this time. Maybe I have indelicate IP “neighbours”…
It could be a net-neighbor (someone inside your netblock) which caused your IP to be blocked. Sadly it’s common practice for many providers to group reputation by netblock neighbors (sad but true).
Another common way to be blocked is to have a website on your server get infected with malware. Some providers are now using malware audits to determine whether they should block mail as well. Basically, an infected site will often send spam without the owners knowledge, so the providers block the IP as a protective measure.
It’s worth looking into the specific error in your “maillog” which should give you an indication as to why Hotmail is blocking you at this time. I believe their error messages are a bit more verbose than others, often providing a URL to where you can learn about the block type itself.
