Frequently Asked Question

Mimecast and the seemingly endless stream of delivery issues to and from them.
Last Updated 3 years ago

Mimecast are an email provider that some companies for whatever reason use for their email. Those who have chosen to use them will have almost continuous delivery issues both incoming and outgoing. We've had so many ticket's opened for reception/delivery issues from and to mimecast customers, and in not a single case was the issue our fault, that we will no longer accept ticket requests on this subject. This is not localised to GEN but the issue is widely discussed in Support Forums and Groups both open and closed.

We will ALWAYS deliver your message to their servers and if at that point the email is rejected then this will be shown in the NDR (non-delivery report) and this indicates the issue is with mimecast. Unlike mimecast our NDR's are detailed and comprehensive.

Looking through a month of tickets I can see common issues occurring over and over again which are.

451 Internal resource temporarily unavailable - We have no idea what or why for this one, seems random and of course delays delivery or prevents delivery depending on the message validity and delivery window.

550 Envelope blocked - This appears to mean the recipient has blocked your email, but when speaking with recipients they have no idea how this happened. Resolving it is often too complex and the recipient often switches to a non-mimecast email as a workaround.

Sending email's with links to "" - this one is a REAL PROBLEM because the senders email instead of being delivered is instead stored by a third party - mimecast and the recipient is forced to 'login' to their system to read their email. There is simply no excuse for this and our advice has and is always to ignore such and DO NOT follow the link and login to a third party system. We have now added an email filter at our gateway to strip out the links from incoming email's.

If your not receiving an email from a mimecast customer then it is for the sender to provide evidence that they are attempting delivery to us and that we're rejecting their email in the form of a SMTP Session Transcript. In mosts cases this will be due to either spam blacklisting or failure in the conversation.

If your reading this and your a mimecast customer who's having issues sending or receiving email then simply pick another email provider and all your problems will go away - guaranteed.

Any and all opinions stated in this FAQ are born from the collective frustrations of helpdesk staff and are solely the opinions of those staff and do not reflect a company policy.

Please Wait!

Please wait... it will take a second!