It appears as though our server has been blocked from sending emails to email accounts that use office365's MTAs.

We run campaigns that send 100's of emails out in a go (to subscribed users), this hasn't been a problem in the past but on Sunday they started becoming blocked.

Does anyone know of a way to get this whitelisted as it's having quite an impact on our business?

I've tried the outlook.com whitelist but they replied saying we weren't blacklisted so I assume that's the wrong one?

Again, these emails are not spam, but promo emails sent to subscribed customers... I hate spam too believe me!

  • I've been running our own email server for two years now and have no issues whatsoever with anything other than two: spam and email involving Microsoft servers. And you/we are not alone but I don't think this is the place to ask this question.
    – Rob
    Jan 17, 2016 at 5:33

2 Answers 2


If your email servers or IP addresses are being blocked it could be for a variety of reasons. There are various organizations and Email blacklist services around the world that exist to prevent spam, mass emailings, virus and worm spreading, and general malicious emailing.

Hostgator has a good writeup on what to do if you're servers or IP gets blacklisted, and how to get it unblacklisted. http://support.hostgator.com/articles/specialized-help/email/email-returned-blacklisted-or-rejected-by-recipient

Most companies sending mass marketing emails have started using services like Constant Contact or MailChimp to handle their newsletter campaigns. These services are trusted by email blacklist services and capable of sending mass emails on a regular basis.


Basically Microsoft's Office 365 servers had decided to greylist us and there was nothing we could do. We actually ended up moving to a hosted service - mailgun in our case which seemed to fix the problem, though we then found that bounced emails were causing us new problems so it's not quite as easy as just throwing money at the problem I suppose.

Shortly before we moved to the hosted service, they greylist was actually removed but we didn't want to risk being re-greylisted.

Advice for anyone else is probably just wait, although it could be a few days :(

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