too large attachment?

Absolutely. Mail servers are setup to reject files that are bigger than 5 MB. Some even less than that. Don't know who you're sending this to. Maybe snail mail is the best bet. Or try IM or even some file sharing service online. Most are not free though.

Most likely the email server you use will not allow attachments that large. Both the smtp (sending) email server and the pop (receiving) email server would have to allow files that large to properly send this message. I don't know of any servers that allow that high of a limit due to easy DOS attacks that could be run on those servers.

Anyway to remove this message you will need to go to /Users/username/Library/Mail/Mailboxes/Outbox.mbox/mbox and delete this file. When you send a new message this file will recreate itself.

With the Mail.app quit and using the Finder, go to Home > Library > Mail > Mailboxes > Outbox.mbox.

Delete the Outbox.mbox folder and empty the Trash.

Launch Mail and if successful, a new Outbox.mbox folder will be created automatically within the Mailboxes folder.

This file is WAY too large to be sent via email. Even if this were possible, most people don't have 180MB of incoming server storage space anyway and the message would be rejected by the recipient's incoming mail server for that reason alone.

A .Mac account has an overall message size limit of 10MB for received and sent messages and most ISPs and email account providers have a similar limit.

In addition, all attachments must be encoded before being sent. The Mail.app uses MIME 1.0 for attachment encoding which is the internet standard. The problem with MIME encoding is it isn't very efficient. The size of a pre-encoded file will be increased by 50% or so by the encoding process.

For example, a 5MB file (pre-encoded) will be roughly 7.5MB in size after the encoding process so even with a 10MB message size limit, you cannot attach a 10MB file (pre-encoded) to a message since a 10MB file will be roughly 15MB in size after the encoding process.