Mimecast
Mimecast is a spam filter, block list, and firewall solution for promoted for businesses, headquartered in London. 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 as spam policy levels and overrides (allow lists).
9 responses across 3 reply codes
SMTP 451
3 responses
SMTP 550
5 responses
1
Mimecast blocked the envelope under an administrative prohibition. Ask the recipient admin to allow your sender if the mail is legitimate.
2
Mimecast anti-spoofing blocked this, often when To and From share a domain but the send goes through an ESP. Adjust spoofing policy or routing with the admin.
3
Mimecast blocked this sender on a manual block list. Ask the recipient admin to remove the block if the mail is legitimate.
4
This Mimecast customer blocked your envelope for their domain only. Ask that admin to allowlist you. It is not a global Mimecast ban.
5
Mimecast rejected this because SPF failed for the Return-Path domain. Publish all sending IPs in SPF, then resend.
SMTP 554
1 responses
Response catalog adapted from the MIT-licensed SMTP Field Manual by ActiveCampaign / Postmark.