Windows 10 Mail can receive but cannot send

Hello, 

If you're facing this issue with Mail app, try re-registering it. Perform these steps: 

1. Search for Windows PowerShell using Cortana or Windows Search. 

2. From results, right click on Windows PowerShell and select Run as administrator. 

3. Then type following command and press Enter key: 

Get-AppXPackage -AllUsers -Name microsoft.windowscommunicationsapps | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register “$($_.InstallLocation)\AppXManifest.xml” -Verbose} 

Let the command complete. 

4. Close Windows PowerShell and see if the issue is resolved now. 

Let us know if this helps!