On the LibreOffice - PreferencesTools - Options - LibreOffice Writer - Mail Merge E-mail tab page, click the Server Authentication button to specify the server security settings.
The outgoing mail server (SMTP) requires authentication
Enables the authentication that is required to send e-mail by SMTP.
The outgoing mail server (SMTP) requires separate authentication
Select if your SMTP server requires a user name and password.
Enter the user name for the SMTP server.
Enter the password for the user name.
The outgoing mail server uses the same authentication as the incoming mail server.
Select if you are required to first read your e-mail before you can send e-mail. This method is also called "SMTP after POP3".
Enter the server name of your POP 3 or IMAP mail server.
Enter the port on the POP3 or IMAP server.
Specifies that the incoming mail server uses POP 3.
Specifies that the incoming mail server uses IMAP.
Enter the user name for the IMAP server.
Enter the password.