Messages returned; Error 550 – Connection Rejected. Your IP is in RBL

This means that the receiving mail server did not accept your message because the IP address of the sender is on a blacklist (usually due to spamming abuse). This can be either your IP address or the IP address of the mail environment that hosts your mailbox.

The good news is that there isn’t anything wrong with your Outlook configuration. The bad news is that you’ll need help from your Internet Provider (ISP) or the service provider hosting your mailbox to get the issue sorted.

Note:
When you get a 550, 553 or relay prohibited error message, then you need to enable authentication for your SMTP server configuration in Outlook.

Check the reported IP address

The first step is to check which IP address is being reported. In most cases, this is either your IP address or the IP address of the mail server that is trying to deliver your message.

To find that out, you can do a “Reverse Lookup”. A website that I prefer to use for this is called MxToolbox as it also allows you to quickly perform follow up lookups to further analyze various DNS, IP and Email related issues.

You can fill out the IP address on their dedicated Reverse Lookup page. The results that are being returned are usually enough to find out whether the IP belongs to your ISP or your mail hosting provider so you can directly contact the correct provider.

Disclaimer:
I’m not getting paid or otherwise compensated by MxToolbox to promote their website or services; I just genuinely prefer to use their website to sort out issues like this and have been using their website for many years now.

Blacklist check the IP address

The next step is to verify which blacklist this IP address is on. Although it is not really needed for you to know this when reporting the issue to your provider, it is still nice to have some “external-proof” which confirms your issue and to add the results to your report.

To do this, you can use the Blacklist Check page of MxToolbox. This will check the IP address you entered against around 100 of the most used Real Time Blacklists (RBLs or DNSBLs) by mail servers around the world.

You can once again simply fill out the IP address obtained from the error message and run the query. A single listing is already enough to get rejected by various recipients as many mail servers use more than one blacklist.

On MxToolbox you can quickly check whether your mail server is blacklisted.

Blacklist check your mail domain

Another approach to find out whether your mail server is blacklisted is to first lookup the MX (mail exchanger) record for your email domain and then blacklist check each returned result.

Once again, we resort to the MxToolbox website as looking up MX records was originally its main function (hence the name).

In the lookup field, you must fill out the domain part of your email address. This is everything that comes after the @ symbol.

For each of the results, click on the “Blacklist Check” link behind it to directly perform a blacklist lookup. As the results will show on the same page above the previous results, you can simply scroll down to perform additional lookups when multiple results were returned.

Note:
It is completely normal for a domain to only return a single result for a domain just like it is completely normal for a domain to return multiple results. However, if you suspect a result to be in error, it is always best to contact your mail provider or administrator.

Blacklist check your own IP address

Another approach to find out whether your own current IP address has been blacklisted is to use the What is My IP? page of MxToolbox.

It will immediately retrieve and show you your current IP address and you can click on the “Get Blacklist Status” button to find out if you are listed somewhere.

When your IP address is listed, this doesn’t necessarily mean that you computer is infected with anything.

As ISPs often randomly assigns IP addresses (via DHCP), it could also mean that you got an IP address assigned of someone that has recently used it for malicious purposes like sending out spam. It could also mean that your IP address is part of a range of IP addresses which has been blacklisted because a large amount of the IP addresses in that range has been recently abused.

In either case, you should contact your ISP so that they can either directly assign you a new IP address or get the IP address or range removed from the reported blacklist(s).