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Email Delays and Order of Receipt

Usually email flows smoothly between users, with delivery times of a minute or less. Like highways, only a certain amount of traffic can go at any one time. At peak times, traffic can clog the highways and cause delays. And sometimes you get stuck behind a convoy of trucks.

This article applies to: Email for Faculty and Staff


Over a half million messages are sent to and through the Cornell email system every day. That system is composed of Office 365 for faculty/staff email, Gmail for student email, Lyris for e-lists, and on-campus mail routing systems that transport the email between these systems. In addition, email often originates outside of Cornell's system, or is routed outside of our system for final delivery.

Microsoft documents a service target for Office 365 of delivering 95% of the email they receive within one minute of arrival at their system. Historically, Cornell has far exceeded that 95% level within the campus routing systems. E-lists can have more variability in delivery times because of queuing within the e-list system designed to protect other email systems from overwhelming blasts of mail from big lists.

If you receive an email that has taken much longer than, say, 30 minutes, to be delivered, you can check for the source of the delay by examining the message headers. See our Display Full Headers pages for the procedure for viewing headers in each email client. If you paste the message headers into a nifty utility, the Email Header Analyzer, you'll see a graphical display of the time spent in every stage of delivery.

If you're the sender of the message and want to know why its delivery was delayed, you'll need to ask the recipient to send you the full headers to investigate.

If delays are excessive in length or frequency, you can open a case with the IT Service Desk. Full message headers are required to investigate delays. Cloud-based email services such as Office 365 and Gmail often do not provide sufficient logging for CIT to investigate without email headers.

CIT will inform you whether the delay was due to a known problem, service congestion, or other cause. Unless the cloud vendor does not meet their Service Level Agreement, there is little recourse beyond reporting the problem to them.

An additional factor that can cause messages to appear "out of sequence" in your Inbox is the geographical distribution of Office 365. Cornell mailboxes are distributed over four different data centers in the United States. For example,

  • Joe sends a message to Fred and Sally.
  • Sally's mailbox is in a data center that is experiencing heavy load, so the message to Sally is delayed.
  • However, Fred (through a different data center) receives the email right away and replies to all. 
  • The reply may take a different "highway" going to Sally, who then receives the reply a noticeable amount of time before she receives the original message.

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