I've got a domain registered with GoDaddy...DNS also there. I deleted the old MX records (Google Apps) and added the new ones (Office365) last Friday at ~2pm. I noticed today that beginning yesterday and going through this morning that some email is still being delivered to the old Google Apps accounts. This is happening for multiple users. For one user, I would say approximately 20 emails were delivered to Google from various sources from yesterday through this morning. During this same period of time, the user received email to Office365 as well.
I did a global check to verify the propagation of the MX records. All the results came back as pointing to Office365. I also went to the GoDaddy account and everything looks as expected. There is only one MX entry in the zone file, for Office365. I called GoDaddy and had them clear out any history or cached zone files. Hopefully that will help. I also deleted the existing Office365 MX record and added it back.
I've changed MX records like this more times than I care to remember and I've never seen an issue like this where the MX records magically revert back two days later.
Anyone seen this before?