We have checked with MXToolbox and we are clean. 

Some gmail users that we send individual messages to directly are not receiving them. We do not receive any return message for spam, etc.

Some gmail accounts go through.

When sending to our son's account at school we get this as well, which may not be related:

ERROR_CODE :550, ERROR_CODE :5.7.1 Your message may not be delivered to this email address in accordance

                                             5.7.1 with GAFE policies..

                                             5.7.1 Thank you - gcdp c41si10054653qta.156 - gsmtp

Any diagnostic tools we can use to figure out why mail is not going through?