An e-mail message with an attachment is sent repeatedly to recipients

When you send an email with a large attachment, the message may go out to the recipients over and over until you delete it from your Outlook Outbox. It never appears in your Sent Items folder.

The cause is usually the connection timeout set for your internet provider’s mail server. When the attachment is too big, the message cannot be transmitted within the time allowed, so Outlook keeps starting the send again from scratch. Either of the following will fix it.

Option 1: Increase the server connection timeout

1. On the Tools menu, click E-mail Accounts.
2. Click View or change existing e-mail accounts, then click Next.
3. Select the relevant account in the list, then click Change.
4. Click More Settings, then open the Advanced tab.
5. Under Server Timeouts, drag the slider to allow a longer connection to the mail server.
6. Click OK.
7. Click Next, then Finish.

Option 2: Compress the attached file

Compressing a file into a zip archive produces a file that is considerably smaller than the original. The formats that shrink the most are text-based ones such as .txt, .doc and .xls, along with image files saved in uncompressed formats such as .bmp.

Source: Microsoft Office Online. (The original article has been removed from the Microsoft site, so the link is dropped rather than pointing nowhere.)