Outlook: Timeout Errors Sending Email
When attempting to send email through Microsoft Outlook, you receive the following error:
Sending ' reported error (0x8004210B) : 'The operation timed out waiting for a response from the sending (SMTP) server. If you continue to receive this message, contact your server administrator or Internet Service Provider (ISP).'
However, the test function in Outlook completes successfully and you are able to ping the SMTP host.
This may occur if your router's MTU (maximum transmission unit) setting is too high for your ISP. Lower the MTU setting in your router.
To determine an appropriate setting, ping the mail host with different size packets until you stop receiving timeouts. Start with 1500, eg:
ping mail.isp.com.au -l 1500
If this times out, try 1400. If that is successful, try 1450, and so on. If 1400 is unsuccessful, try a lower value such as 1300.
This issue may occur suddenly. This is because Internet Service Providers routinely reconfigure, replace, and upgrade their equipment, and settings which worked one day may not work the next day.