Common bounce errors
550 sender has failed strict spf –
The sender/FROM domain fails strict SPF checking. The FROM domain does not pass spf.
550 This message was classified as rSPAM and may not be delivered –
The message has been marked as spam and not delivered.
550 This message has a blank subject –
The message was sent with out a subject.
550 err commondomain This message from domain does not pass SPF checks –
The FROM domain is a common domain and does not pass SPF checks. Common causes are a FROM domain of a common mail provider like @gmail
550 recipient domain does not resolve to any MX –
The recipient domain does not resolve to any Mail server.