Issue: I can receive emails but I cannot send emails; error message “Send/Receive Error” or “Sending 1 of 21 emails“. The Outbox folder has 21 unsent emails.
Solution 1: Check your Deleted Items folder and empty any messages in it.
If the messages are too many such as 25000, log into the your Outlook online account and then delete the items from there since it faster that way.
NB: If you have any other folder with more than 1000 email messages try to see if you can archive some of the messages or delete the unnecessary ones. Ideally keep your email folders under 1000 messages to reduce burden on the email client since it has to upload all those emails when you open it. The delay in uploading the messages could prevent you from sending or receiving emails.
Solution 2: Create a new Outlook profile by going to Control Panel\All Control Panel Items, then click Mail > Show Profiles and the click on Add. Now enter a name for your new profile such as New Outlook Profile and then set it as the default profile by selecting Always use this profile and then click OK. Restart Outlook to see if the issue has been resolved.
Solution 3: Try deleting the messages in the Outbox folder and then restart Outlook to see if you can send or receive emails.