I can't send mail. Receive yes!

try using the connection doctor in Apple Mail

https://support.apple.com/en-us/HT204173

If you can’t send emails in Mail on Mac

• Make sure the outgoing (SMTP) mail server is specified as directed by the provider of your email account.
• Check for a message indicating you need permission to send an email (due to parental controls).
• Choose Window > Connection Doctor, then follow the instructions displayed in the Details column.
• If there’s a firewall problem, verify that firewall software isn’t blocking email traffic on port 25, 465 or 587. Also check whether you’re affected by another firewall, such as software installed separately from macOS or built into an Internet sharing router, or administered by your email account provider. Contact the appropriate apple support person for information.

Regards,

Adrian

[Link Edited by Moderator]

Hi, have you changed your eMail password lately?

Works sometimes even if you haven’t

Open Keychain Access in Utilities, then either check the Password under that item, change it, or delete it and start over.

Enter the part after the @sign in the search bar.

You may have duplicate or more entries in Keychain Access.

If using a browser to login via WebMail works it's not Name or Password, but likely old passwords interfering.

There''ll be at least 2 entries, maybe more.

The trouble is that Mac Mail almost always chooses the old Passwords if you don't change or get rid of the old ones.

Writing an effective Apple Support Communities question - https://discussions.apple.com/docs/DOC-5931

What email client? What model computer/year? What error messages? What OS? What happens when you try?

Writing an effective Apple Support Commun… - Apple Community