split certificates question

Please excuse the ignorance i'm new to all this.

Trying to get a server running as we are moving to non hosted emails . 

I have the mx records in the domain setup and that seems to work . I setup letsencrypt as per a blog post that's all good . I then setup an a record to point to my local network but chrome refuses to connect as adding mail. in front of the domain then makes the cert common name incorrect. 

any help would be very appreciated 

regards Jon