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CU's Exchange (Office 365, Outlook) system limits the size of email messages, including their attachments, to 25 MB. CU's e-lists limit full message size to 1MB.

Messages that exceed the size limits are rejected by Exchange. Note that email rejections can also occur as a result of CIT's efforts to block spam.

Cornell staff considering sending attachments (large or otherwise) have options, including:

Posting the attachment to a web site. An example is using CIT's wiki service where one can restrict access to individuals or to just the Cornell campus, as necessary.
Using Cornell's DropBox service available to faculty, staff, and students (among others) who need to send files. Using this service is also more secure than sending most attachments via email. With this service, anyone with an active Cornell NetID may send and receive files.
- Files can be sent to non-Cornell email addresses using this service. In fact, you can whitelist an email address, granting that email address-holder privileges to send files, too. You manage the duration of access and the password for the whitelisted email address.
- Maximum size specified in the service's FAQ.
- DropBox Instructions, courtesy of ILR staff. Note: These instructions were created when the DropBox service was hosted by the Registrar's office so some elements will be different.
Using Cornell's DropBox service component that is part of Cynergy. Available to those with functioning NetID privileges. Has an entire workflow component, so you can verify that the doc. has been picked up, for example.

CIT's information on attachment size limits

What's the biggest attachment I can send?

CIT limits the entire message's size, not just the size of specific attachments, to 25 MB. The email message itself, as is any attachment encoding overhead data, are all part of the total message's size.
Rejected message from CIT's e-list servers inform the sender that the message was rejected.
Mailing lists that exceed x MB of messages within y days are automatically suspended. This is a cumulative total, not due to any single message, so avoid sending attachments to mailing lists! (11/4/08: x = 100MB, y = 7 days.)

Here is the specific answer if using CIT's email.

http://www.it.cornell.edu/services/cmail/faq.cfm#cmail-limits

Limited information specific to the limits related to CIT's e-list service

https://it.cornell.edu/lyris/send-large-files-e-list-messages
- As of 1/27/17: E-list messages have a maximum size of one megabyte, including attachments. It helps to know that the very act of attaching a file to a message increases its size, so if your attached file is over, say, 700K, it's going to be too big.