Email servers work together in a giant relay of messages all across the Internet. They generally use the SMTP Protocol and POP3 Protocol to communicate between each other and with individual users/mailboxes. Often users may want to send attachments that are large and generally exceed the limitations set within their own local SMTP Relay Server or that of the Recipient's Mail Server.
Email in general is limited to 5-10 MB per message, including attachments. When a message is sent exceeding this limitation the email message may not be delivered to its final destination or may not even leave your Outbox at all.
Depending on the situation you may receive a Failure Receipt from the recipient server or your SMTP Relay Server indicating that the message you sent was too large and can not be delivered.
If the message is "stuck" in your Outbox and will not go away you may need to restart your email program and attempt to delete the message from your Outbox or move it into Drafts BEFORE your computer begins to attempt sending it again. Once it starts to send, often you will not be able to remove the "stuck" message from your Outbox or delete it.
If the messages goes from your computer
to the SMTP Relay Server but you are getting an email back saying it could not be delivered you will need to find another method to re-send the attachment. One way to accomplish this is to upload the file to a file hosting site online and instead only send a Link or Reference directly to the large file via email. One example of this is the site: http://www.transferbigfiles.com/