question about certificates usage

[alice]

may I ask another question I am not sure.

I have got the certificates from letsencrypt for the root domain, in 
this case it's coakmail.com

since the MUA uses coakmail.com as smtp/imap servers, this has no problem.

but my MX RR is: box.coakmail.com

I know MX only accpets messages on port 25 (I am right here?), so the 
certs issued to root domain will have no side effect for the incoming 
messages from other MTAs?

Thanks.