unable to send emails suddenly, although receiving them, any suggestions?

If you're unable to send emails while still being able to receive them, and your emails are stuck in the Outbox, there are a few potential causes and solutions you can try:

1. Check for Stuck Read Receipts: Sometimes, a read receipt can get stuck and cause issues with sending emails. This may lead to Outlook continuously trying to send a message that doesn't exist. You might want to check if there are any read receipts in your Outbox and delete them if necessary.
2. Metered Connection Warning: If you're on a metered connection, Outlook may restrict sending emails. Ensure that you are connected to a network that is not metered. You can check this by selecting File in Outlook and looking for any warnings related to your connection.
3. Repair Outlook: You can try repairing the Outlook application. This can often resolve issues related to sending and receiving emails. You can find the repair option in the Control Panel under Programs and Features.
4. Check SMTP Settings: Ensure that your SMTP settings are correct. Incorrect settings can prevent emails from being sent. You can verify these settings in your account settings in Outlook.
5. Third-party Applications: Sometimes, third-party applications can interfere with Outlook's ability to send emails. Use Process Monitor to check if any applications are accessing the MAPISVC.inf file, which may cause issues.

If these steps do not resolve the issue, consider contacting your email service provider for further assistance.