If you have a custom email address (firstname.lastname@example.org), and you can receive but not send email, the reason may be that your internet service provider (ISP) is blocking you from sending email. You will need to call your ISP’s tech support and find out what their “SMTP email server settings” are, and then go back into your email software’s account set up screens and replace the SMTP settings for your new email account with the SMTP settings from your ISP.
SMTP settings will include a server name (aka host name), an account name, password, and possibly a port. Valued Websites can’t really help you get this information from your ISP — you’ll just need to wait on hold with your ISP’s tech support until you can get a person on the line who knows what ‘SMTP settings’ are — not all of them do.
If they insist that you don’t need to use their SMTP servers, then ask them if they need to unblock your custom email address on their end.
If you can’t send or receive email, then you probably set up your email software incorrectly, or your internet connection isn’t working right now. In this case try the account setup process again in case you made a typo.