When a message is not delivered to the recipient's server, two scenarios: The error is…
When you send an email containing a large attachment, the message may be sent multiple times to recipients until you remove it from your Outlook mailbox. The message never appears in your Sent Itemsfolder.
The problem may be related to the delay in connecting to your Internet service provider’s email server. If the size of the file attached to the email is too large, the message cannot be sent within the allotted time. Therefore, it is sent again regularly. To solve this problem, apply one of the following options:
Option 1: Increase the time limit for connecting to the server
- In the Toolsmenu, click Email Accounts.
- Click View or edit existing email accounts,then next.
- In the list, click on the appropriate email account and thenedit.
- Click On Additional Settings,then on the Advancedtab.
- To increase the maximum duration of connections to the email server, under Server Delays,drag the cursor.
- Click OK.
- Click Next,then finish.
Option 2: Compress the file attached to the message
By compressing a file into a zip file, you’ll get a file that’s significantly smaller than the original file. The smallest types of files after compression are text-based files such as .txt, .doc, .xls files and graphics files that use uncompressed file types such as .bmp.
Source: Microsoft Office Online