Not sure if this is the correct forum to post this, but bear with me.
We are having a problem with certain domains sending emails to our domain
name. Most emails come in just fine, but there are a handful that are not
able to send to us. Those that fail recieve an undeliverable with the
following error: 451 ourdomain.com name server timeout

Does this have anything to do with reverse DNS lookup?

Thanks,
Jason