problem with outlook , can receive emails but can't sent

If Outlook.com can receive mail but cannot send, check these common causes in this order:

1. Check Microsoft cloud storage. If storage is full, Outlook.com cannot send or receive email. Messages sent while storage was full can bounce back and are unrecoverable. Check mailbox storage and remove items if needed.
2. Check the account status. If the password was changed recently, update it. The account can also be temporarily blocked because of unusual sign-in activity. If sign-in is failing, use the Microsoft account sign-in helper. If the account is blocked, use the Outlook.com account unblock steps.
3. If the problem happens on only one device, remove that device’s partnership with the mailbox. This often fixes sync issues for that device.
4. If using Outlook on the web, verify the sending address. Go to Settings > Mail > Sync email and check the From address. Then go to Settings > Mail > Compose and reply and reset the default From address. After that, clear the browser cache or try another browser, then sign out and sign back in.
5. If using Outlook for Windows instead of Outlook.com in a browser:

• Make sure the PC is connected to the Internet by opening a browser and testing a site such as Microsoft or Bing.
• In classic Outlook, check the status bar. If it shows Disconnected, Working Offline, or Trying to connect..., go to Send/Receive > Work Offline to switch back online.
• Check the Outbox for a stuck message with a large attachment. If an attachment is larger than 20 MB, remove it and resend with a smaller file or a link.
• If antivirus email scanning is enabled, it can block sending or receiving.

References:

• Can't send or receive email in Outlook.com
• I can't send or receive messages in Outlook
• I can not send emails but I receive them - Microsoft Q&A