I know this was listed as "resolved" and it is old, but couldn't actually tell that it was "resolved."
Don't know if this is your issue, but this resolved the problem for us
We are having the same problem with 2.6.2 with users in our domain not receiving emails. We first tested to see if the moodle server would email domains like gmail.com and outlook.com, etc. We confirmed that it was sending the emails. Might want to test that as it lets you know for sure that the moodle server IS working.
The problem was resolved when I noticed that we had changed the default support email. Our anti spam software didn't like an external server sending emails to our domain as it viewed them as email spoofing i.e. a forged email address. Changing the support emails domain to anything else fixed the problem.