Something I'm noticing, which is probably more of an Outlook issue than anything else, is that Outlook is inconsistent in where it tries to find Autodiscover settings.
Our mail server is https://mail.mbsportsweb.ca
. There's 500+ domains in there and we don't
add the other domains into the bindings for IIS. One such domain is oakvillerangers.ca. We control the DNS for all these domains, so we've created SRV records to point autodiscover requests to mail.mbsportsweb.ca. When using the MS Autodicover Connectivity Tester, it goes through and eventually finds that SRV record and passes its test.
Trouble is, that is appears Outlook 365 doesn't always follow the path to use the SRV record. Adding accounts from within Outlook itself seems to be hit-and-miss (mostly miss), but, we can add accounts using the old Mail Setup dialog found within the old-school Control Panel of Windows 10. Adding the account that way, even using the "Office 365" option (which makes no sense) seems to work every time.
Then after doing that I'll open up Outlook and run Fiddler and notice that it's constantly making requests to autodiscover.oakvillerangers.ca. Why? I have no idea - the account is already setup and seems to be working. Originally I didn't create a DNS record for autodiscover.oakvillerangers.ca and it looked like Outlook was constantly updating the account, so I created a CNAME to point to mail.mbsportsweb.ca. That seems to have stopped Outlook from saying it's constantly updating the account, but, looking at Fiddler I still see fairly frequent requests happening to autodiscover.oakvillerangers.ca.
So while things seem to be working, I'm not sure I fully understand how things are supposed to be setup. Is autodiscover.domainname.com
a requirement? Who's requirement? Can it be a CNAME or must it be an A record? In SmarterMail, we have the "hostname" for each domain set to be mail.mbsportsweb.ca (which seems to be the best way to get the proper XML returned from an autodiscover request) - is that correct? We used to have these all set as mail.domainname.com
(and we have CNAMEs setup for those too) but that didn't seem to work.
Too many variables all at once here - from what is the proper DNS settings to the proper Hostname settings in SmarterMail to bindings in IIS, etc.