Mimecast
Mimecast is a spam filter, block list, and firewall solution for businesses. Mail is routed through mailservers owned by Mimecast (*.mimecast.com) before it's passed along to the recipient domain's mailservers for final delivery. The recipient is able to manage a lot of delivery settings such a allow lists, so it helps to ask the recipient to run the described debugging tests in the user guide to identify and resolve delivery issues.
Codes
- 451↑
- 451 Open relay not allowed - https://community.mimecast.com/docs/DOC-1369#451 [#]
- 451 Internal resource temporarily unavailable - https://community.mimecast.com/docs/DOC-1369#451 [#]
- 451 Invalid Recipient - https://community.mimecast.com/docs/DOC-1369#451 [#]
- 550↑
- 550 Administrative prohibition - envelope blocked - https://community.mimecast.com/docs/DOC-1369#550 [#]
550 Rejected by header based Anti-Spoofing policy: email@example.com - https://community.mimecast.com/docs/DOC-1369#550 [#]
Often this bounce is related to sending both 'To' and 'From' the same domain but through a third party (like an ESP).
- 550 Rejected by header based manually Blocked Senders: email@example.com - https://community.mimecast.com/docs/DOC-1369#550 [#]
550 Envelope blocked - User Entry - https://community.mimecast.com/docs/DOC-1369#550 [#]
This is not a global block for all Mimecast customers. Instead just this single recipient domain's administrator has blocked your mail.
550 DMARC Sender Invalid - envelope rejected - https://community.mimecast.com/docs/DOC-1369#550 [#]
The inbound message has been rejected because the sending IP address isn't listed in the published SPF records for the sending domain (Return-Path header domain). Ensure all the IP address for your mail servers are listed in your SPF records.