I can receive emails but cannot send them. How can I fix this?

If you are unable to send or receive email from the native Mail App, you will usually see an error message at the bottom of the Mail sidebar. This will usually indicate the nature of the issue.

As you report difficulties with sending email, but can receive, your incoming (IMAP?) Mail Server settings will be correct; you will need to check/verify your SMTP (Send) Mail Server settings for the affected email account. Very often, the associated Username or Password will be incorrect or require reset.

Settings > Mail > Accounts > [Your Email Account] > Account > SMTP > [Primary Server] > [Outgoing Mail Server] - check these settings

If needed, you may need to check these settings with your email provider.