How to test MX resolution?

Your domain suddenly stops receiving email. Before you panic, the first thing to check is whether your MX records are actually resolving the way you expect. This is a quick check that takes about two minutes.

Command line tools

If you're on Mac or Linux, open Terminal and run:

dig MX yourdomain.com

On Windows, use Command Prompt:

nslookup -type=MX yourdomain.com

You're looking for an ANSWER SECTION that lists your mail servers with priority numbers. Something like:

yourdomain.com. 3600 IN MX 10 mail.yourdomain.com.
yourdomain.com. 3600 IN MX 20 backup.yourdomain.com.

No answer section at all? That's your problem. Your domain has no MX record pointing anywhere, which means inbound mail is going nowhere.

Online tools (no terminal needed)

MXToolbox is the most widely used option. Type your domain, hit MX Lookup, and you'll see your records plus whether each server is reachable. It also flags common problems automatically, which saves time if you're not sure what you're reading.

The Google Admin Toolbox (Check MX) is another solid choice, especially if you're on Google Workspace. It runs a broader set of diagnostics in one click.

What to look for

• Correct mail servers. The hostnames should match what your email provider gave you. A typo here (one wrong character) means zero inbound mail.
• Priority values that make sense. Lower numbers get tried first. Your primary server should have the lowest priority number. If all your records have the same priority, that's fine for simple setups.
• Servers actually respond. MXToolbox will flag a timeout or connection refused. An MX record pointing at a server that won't accept SMTP is just as broken as no record at all.
• Consistent results from different locations. DNS changes take time to propagate. If you test from your office and it looks fine but a sender in another country gets a bounce, try a geo-distributed checker like DNSChecker.org to see what different regions are seeing.

One thing that trips people up

So an MX record can resolve perfectly on paper but still fail to receive mail if the listed server is blocking connections on port 25. The record lookup itself won't catch this. MXToolbox's SMTP test goes one step further and actually tries to connect, which is worth running if the basic lookup looks clean but mail still isn't arriving.

But if you're seeing something unexpected and can't work out whether it's a DNS issue, a server issue, or something else entirely, you're welcome to bring it to our SOS hotline. Free, no pitch, just help.