If you would need an A record for it too, just enter it like this in DA:
mail 14400 A xxx.xxx.xxx.xxx
where xxx stands for the ip address of the external server.
Ah, sorry, you're correct.
I forgot that because I only use "mail" there (without the quotes), no full domain name, but that's for internal use. When stating the full domain name it needs a dot at the end on the right side too indeed.