Google

smtp;421 4.7.0 [x.xx.xx.xx] Our system has detected that this message is suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information. - gsmtp

High severity

smtp;421 4.7.0 [x.xx.xx.xx] Our system has detected that this message is suspicious due to the nature of the content and/or the links within. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information. - gsmtp

High severity

421 4.7.28 [x.xx.xx.xx] Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines. - gsmtp

High severity

smtp;450 4.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered. Please resend your message at a later time. If the user is able to receive mail at that time, your message will be delivered. For more information, please visit https://support.google.com/mail/?p=ReceivingRate - gsmtp

Medium severity

smtp;451 4.3.0 Mail server temporarily rejected message. - gsmtp

smtp;452 4.2.2 The email account that you tried to reach is over quota. Please direct the recipient to https://support.google.com/mail/?p=OverQuotaTemp - gsmtp

Low severity

smtp;550 5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces. Learn more at https://support.google.com/mail/?p=NoSuchUser - gsmtp

smtp;550 5.2.1 The email account that you tried to reach is disabled. Learn more at https://support.google.com/mail/?p=DisabledUser - gsmtp

smtp;550 5.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered. For more information, please visit https://support.google.com/mail/?p=ReceivingRatePerm - gsmtp

High severity

smtp;550 5.7.1 [x.xx.xx.xx] Our system has detected that this message is likely suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information. - gsmtp

Critical severity

smtp;550 5.7.1 [x.xx.xx.xx] Our system has detected that this message is likely unsolicited mail. To reduce the amount of spam sent to Gmail, this message has been blocked. Please visit https://support.google.com/mail/?p=UnsolicitedMessageError for more information. - gsmtp

Critical severity

smtp;550 5.7.1 Unauthenticated email from example.com is not accepted due to domain's DMARC policy. Please contact the administrator of example.com domain if this was a legitimate mail. Please visit https://support.google.com/mail/answer/2451690 to learn about the DMARC initiative. - gsmtp

High severity

smtp;550 5.7.1 [x.xx.xx.xx 14] Messages missing a valid address in From: header, or having no From: header, are not accepted. - gsmtp

smtp;552 5.2.2 The email account that you tried to reach is over quota. Please direct the recipient to https://support.google.com/mail/?p=OverQuotaPerm - gsmtp

SMTP; 552-5.2.3 Your message exceeded Google's message size limits. Please visit http://mail.google.com/support/bin/answer.py?answer=8770 to review our size guidelines.

SMTP; 552-5.7.0 Our system detected an illegal attachment on your message. Please visit http://mail.google.com/support/bin/answer.py?answer=6590 to review our attachment guidelines.

High severity

smtp;553 5.1.2 The recipient address <example@example.com> is not a valid RFC-5321 address. - gsmtp