Are you able to access the spam filter policies for office 365? I think the solution may be in there. Since the from address is always different and it only happens to some office 365 account, gives me a strong suspicion that the problem might not be the email module.
Did you see the documentation about Sending Policy Framework? https://docs.mendix.com/deployment/mendixcloud/sending-email
See for office 365: https://technet.microsoft.com/en-us/library/dn789058(v=exchg.150).aspx