Onega’s Mimecast FAQ - Frequently Asked Questions

Q: Why is some email delayed when it comes in via MimeCast?

A: Mimecast has many tools which stop spam coming into your mailbox. One of these uses a concept called ‘greylisting’ which recognises that many spam mailing systems are set so that if they can’t deliver email to you immediately; they will simply move on to the next recipient. This is in contrast to normal email systems which have a ‘retry schedule’ and will try again and again to deliver email until a timeout occurs (normally email might retry for a few days before giving up finally and returning the sender an NDR- Non-Delivery Report) message.
Mimecast will thus ‘greylist’ email from unknown (new) senders or sources and will accept the resent email after a minimum of 60 seconds from the original email attempt. Once email communications are established with the sender, the address is recognised in future and not greylisted. Manual accept entries can also be added if needed for any given address should that be needed.
NB - the exact time of the delay is down to the sender’s email system and how that is configured. Some will try again after 5 minutes if they can’t get through first time, others 20 minutes, and others may be less often. The sender’s IT department (and / or their mail provider) decide this.

Two more things to note here are that 1) when an email comes in, the timestamps of sending may be from a different timezone and 2) if you are waiting for an email from someone and it is possibly greylisted & urgent - if they resend (manually forward the mail to you or send a new mail): then this will normally get through immediately even if their email system is still waiting 60 minutes to retry.

Possible other reasons for email delay (or perceived delay) include writing an email while not connected to the Internet (ie on a phone in a tunnel etc.) - which will then normally be sent when reconnected to the Internet again, and .. sometimes systems get very busy or have temporary faults so while most emails are delivered in seconds, you can’t always assume that to be the case. Email (as for the post and as for life) is not perfect…

>> If there are any other questions you would like answered, please do let us know (quickest via the form here) and we’ll endeavour to add these.

If you need any help on Mimecast, please do give us a call on the Onega Helpdesk and we’ll be happy to assist.

Thanks to Paul Hudson via Flickr for the header image on this page.