this issue can be caused by several factors, such as Internet connection, Outlook settings or corrupted outlook profile. We can try the following methods to see if the problem can be resolved.
Click the “Send/Receive” tab, then click “Send all”.
Method 2: New Outlook profile
1. Exit Outlook.
2. Go to Start > Control Panel, click or double-click Mail.
Mail appears in different Control Panel locations depending upon the version of the Microsoft Windows operating system, Control Panel view selected, and whether a 32- or 64-bit operating system or version of Outlook is installed.
The easiest way to locate Mail is to open Control Panel in Windows, and then in the Search box at the top of window, type Mail. In Control Panel for Windows XP, type Mail in the Address box.
Note:The Mail icon appears after Outlook starts for the first time.
The title bar of the Mail Setup dialog box contains the name of the current profile. To select a different existing profile, click Show Profiles, select the profile name, and then click Properties.
3. Click Show Profiles. Choose Prompt for a profile to be used.
4. Click Add.
5. Type a name for the profile, and then click OK.
6. Start Outlook, and choose this new profile.
7. Try to send the emails in the new profile