Will MX records with a longer TTL help mitigate effects of DoS attack? [duplicate]

Q: Would setting a longer TTL help mitigate future problems like this?

A: Only for those hosts that have your MX record cached before the attack occurs. Any hosts that don't have it cached will need to look it up and will not be able to.

As Joequerty's already said, increasing the TTL can help as long as the sending servers already have your records cached. However on the flip side, if you need to update your MX records for some reason in the future, the more you increase the TTL the longer you'll have to wait before everyone will have picked up those changes. It can be a balance between the two and you have to decide which is more likely to happen, and in the event of it being required, which is likely to cause the most downtime.

Hopefully these two events are the exception rather than the rule, but personally I'd be thinking more about moving the domain to a more robust host than adjusting the MX records.