The first advice is that email contains no guarantee of delivery. The second is the hoops that must be jumped through to get mail delivered multiply daily as corporation attempt to regulate something governments (most especially the US government) simply refuse to touch (Political donations) Have you asked the mail providers that are not accepting your mail why not? Are you receiving non delivery notifications? (your supposed to by most are turned off because of spam) Often mail is bounced because of SPAM content (such as signatures the receiver does not like). At one point Yahoo refused delivery of mail that contained a valid reply to header ( it was a spammy thing apparently) Have you checked that the SMTP server being used is actually Googles? Apparently lots of people try and send mail from a server (say comcast) and get their mail rejected as it is a gmail address sending from somewhere completely different.