With my old DNS service I redirected m.domain.com to mail.domain.com.
It seems like Dynu doesn't allow aliases shorter than 3 characters.
Why is that?

Could you please let us know the exact DNS record that you want to add so that we may add it for you?

The issue with aliases not allowing more than 3 characters has been reported to our developers who will fix this in the next website release.

We apologize for the inconvenience. Thank you for your patience.