just thought i’d throw this out into the interwebz since it seems to be such a well known no-can-do that is actually a very handy can-do (where applicable)…
DNS admin warnings advise against doing a “bare” (no prefix) CNAME along with an MX record, example
What i’m successfully demonstrating here is clearly non-standard according to the specs…
however, i’ve proven it does work for SOME servers implementation of the standards (including major provider Office 365 on the MX side) so it’s worth trying with your servers if this provides a convenient solution for your needs… and it will be immediately verifiably working or not; no “sometimes” ambiguity to worry about.
Very simple – just try positioning the MX record ABOVE the CNAME in the record order, e.g.
along with a happy O365 MX record (sorry i can’t slam dunk it with showing actual host names, you’ll have to trust me that it really did work and i didn’t make this up)
I’ve found this “MX ToolBox” to be loads of help for double checking the various DNS responses: