Plain text or HTML. In the case of HTML, an alternative plain text version of the body is automatically created and attached to the outgoing message, as expected by contacts who can't or don't want to use an HTML-capable program to read their mail.

Replies can be initiated in plain text, HTML, or the same format than the original message, the latter being assumed to be the best choice in general. In any case, the format can be changed manually inside the composer during the edition of the reply.

If set to "Yes", refuse addresses that are seemingly incorrect as input. Even though they are technically correct, certain syntaxes for email addresses are not actually used nowadays, and are more likely to be input errors than intentional. Examples: a bare word without any @-sign and domain name, or a group (ending by a colon and semi-colon). When this setting is set to "No", the responsibility of rejecting a wrong email address is left to the rest of the email system. Generally a message sent to a wrong addresses will result in a non-delivery notification by the mailer daemon.

Composing a new message or a reply can happen outside of the user interface, in an external editor. This setting indicates the name of that program, optionally including a path. The program will received as an argument the path of a temporary file containing the current message contents.