dns subdomains

[John Curtiss]

say I bought domain.com from godaddy. but I only ever want to use the subdomain a.domain.com. I want my office365 email to come from @a.domain.com, not @domain.com. I want to browse to http://a.domain.com, or even http://www.a.domain.com, but never http://www.domain.com. 

when I tell office365 about the a.domain.com domain, it wants to add dns records, including mx and txt. but obviously I need a cname or an A record for a.domain.com as well, and godaddy's dns won't let me have an A record or a CNAME record called "a.domain.com" when there's an MX record or a TXT record named "a.domain.com". if I change the MX and TXT records to "mail.a.domain.com", I can then add a CNAME record for a.domain.com. but office365 says DNS is broken now. it is expecting the MX and TXT records to be named a certain way. and it does not appear to even recognize that mail.a.domain.com exists. 

for testing purposes, to see what the DNS records looked like, I added just 'domain.com' to office365 as well. the MX and TXT records generated both had "@" as their hostnames, which I guess is some special symbol, because there is also an A record named "@", and I didn't get any error about that conflicting with the MX or TXT records named "@". 

is this the expected behavior for subdomains? is godaddy's rule about having a CNAME with the same name as an MX record typical of DNS hosters? is mail going to work if I have MX and TXT records for "mail.a.domain.com" even though office365 is warning me that it won't?

[#2]

You are talking about two different things now, GoDaddy DNS records and Office 365 domains.

To get mail flow work in Office 365, you need to add the domain to Office 365 and confirm that you own the domain. This is typically done by TXT record with value like ms=MS11111111. If you're using GoDaddy, I would suggest that you'd do this manually instead of letting Office 365 to do it automatically.

So, lets say you wan't to use ONLY abc.domain.com in Office 365.

1.  You need to add the domain to Office 365. After that, it shows you the TXT record you need to add to GoDaddy. Note! Make sure that you DO NOT have domain.com registered in Office 365.
2. Go to GoDaddy and open domain.com DNS management.
3. Add a new TXT record with hostname abc with the ms=MSxxxxxxx generated in step 1.
4. Go back to Office 365 and complete the validation

Now the domain is validated and you can add the rest of the records, such as MX and SPF to enable the mail flow. You can also add CNAME records as needed.

[#3 · Accepted solution]

Thanks for your reply. I figured it out after. Turns out this was the key question:

"is godaddy's rule about having a CNAME with the same name as an MX record typical of DNS hosters?"

And the answer is yes. DNS RFC says I can't/shouldn't have a cname record for a.domain.com and an mx record for a.domain.com at the same time. I have to use an A record instead of a cname.

I let office365 create all the records for a.domain.com, and it created them correctly. I just really (thought I) wanted that cname for a.domain.com. But it turns out that I don't really need it-- I can create the cname www.a.domain.com, and godaddy's dns will also allegedly let me "forward with masking" a.domain.com anywhere I want while keeping a.domain.com displayed in the address bar.