Why does email deliver to one recipient but get rejected for another with Mimecast?

Email marketer from Reddit explains that the organization receiving the email might have internal email filtering rules or policies. They may be configured differently for different groups of users.

Email marketer from EmailGeek explains that Mimecast considers the sender's reputation. A sender with a low reputation score may have emails rejected for some recipients but not others, depending on how aggressively each recipient's organization filters based on reputation.

Email marketer from Information Security Stack Exchange explains that actions will depend on policy (and Mimecast being a security gateway, you might want to inspect the log for specific policy hits, even if I'm not a Mimecast user myself).

Email marketer from EmailDiscussionsForum shares that The content of the email might trigger different spam filters for different recipients. The email may contain words or phrases that are more likely to be flagged as spam for one recipient than another.

Email marketer from Quora explains that Y and X might have their Mimecast rulesets configured differently. Therefore, Y might be blocking more content than X.

Email marketer from StackOverflow shares that one recipient may have added the sender to their personal allow list (whitelist), while the other has not. This can bypass spam filters.

Expert from Word to the Wise explains that if one recipient marks an email as spam or unwanted, Mimecast may use this information to block similar emails from reaching other recipients within the same organization. This is a form of user-based filtering.

Expert from Email Geeks explains that if Y marks the email as spam, it will go to the spam folder, even if the sender has a history with others at the domain

Expert from Email Geeks shares that X might have a personal spam threshold set differently than Y. Mimecast allows end users to adjust settings.

Expert from Email Geeks shares that it's possible that X was communicating and Y had mail trapped in their mail quarantine, instead of allowing to deliver they could have manually bounced or blocked it.

Expert from Email Geeks explains that it helps when you are successfully mailing to X, Z and B, D and E and then you add Y. Y is more likely to get the mail in their inbox because you have a history with others at that domain.

Expert from Email Geeks shares that a history of emails being appreciated by users can help delivery, at least until it doesn't.

Expert from Email Geeks explains that X is interacting with the email, replying to it, while Y isn't, which can cause mail filters to treat the two situations differently.

Documentation from Mimecast explains that Mimecast uses sender feedback loops. If a recipient marks an email as spam, Mimecast can use this information to block future emails from the same sender to other recipients within the organization.

Documentation from Mimecast explains that Mimecast allows individual users to set their own spam filtering thresholds. One recipient may have a stricter spam policy than another, leading to different outcomes for the same email.

Documentation from Mimecast shares that If a recipient has previously blocked a sender, Mimecast will reject future emails from that sender, even if other recipients haven't blocked them.

Documentation from Mimecast explains that Mimecast has connection limits from single sending IPs. If these limits are exceeded the email can be delayed or blocked for some users