Our client recently switched to Office 365. Previously his email was hosted with his domain. He runs a web store and each time people order something or ask for info he receives an email about it. We had to adjust the PHP code of course in order to make it work with the smtp settings of Office 365.
Ever since the switch we noticed not everything is received by email when using the website forms or placing orders. We tested all email addresses with testconnectivity.microsoft.com and found no problems. We checked the Mail flow in Exchange and the missing emails are nowhere to be found. We double checked the SPF records to be sure, everything is ok.
Is Microsoft forbidding you to use PHP mail with Office 365?