Alternative Fastmail nameservers ?
 

I'm having a few issues with my domain name registrar. The domain is hosted by Fastmail (using the ns1. and ns2.messagingengine.com nameservers).

I wanted to try updating the nameservers with my registrar (using their control panel) to see if it would actually be actioned.

I note that ns1.fastmail.fm and ns2.fastmail.fm appear to exist, plus there also seems to be fmns1.messagingengine.com and fmns2.messagingengine.com.

I imagine these are aliases of the two main nameservers rather than being unique alternatives.

However, I'm wondering whether anything hideous might go wrong if I were to temporarily use them?

It can take up to 72 hours for nameserver changes to fully propagate. Not sure what the other FM nameservers are for, but they definitely say to use the ns1 and ns2 messagingengine.com ones. What are you trying to test?

If you don't know what you're doing, twisting knobs with a production system to see what happens probably won't help and will hurt, a lot. The DNS is an amazing system, but it's not at all forgiving of configuration mistakes or misunderstandings. And most registrar tech support staff don't even understand it.

They aren't, fmns1 and fmns2 are authoritative for FM's own domains, not for hosted customer domains.