By default, only the user id is used when delivering email messages. Using this check box, the user name can also be used. Spaces are replaced by dots. For a user account "jack" with full name "Jacques Gelinas", email may be sent to either jack@the_domain or jacques.gelinas@the_domain.
While this sounds appealing, it is not a recommend option. It is possible within your organisation to have two persons named identically. Unless you rename the first account, the older user will receive messages intended for the new user. The proper solution is to use only the account name, which is unique, so correspondents can't blindly guess the proper email address.
Relay control lets you decide who may use your SMTP server (sendmail) to relay messages. Spammers are often borrowing SMTP servers to mass-distribute a message to a huge list of people. By enabling relay control, you can stop this from happening. Unfortunately, misconfiguring relay control rules may mean that some of your users won't be able to send email.
This check-box allows you to enable/disable the rules. A new sendmail.cf file is generated, without losing the rules configuration.
Note that this check-box DOES NOT control the "rejected senders" feature. "Rejected senders" allows you to reject email from well-known spammers. It does not limit the ability to relay.
It is a way to reduce spam. Check out http://www.ordb.org to learn about this feature. For every mail received, a special server is queried to accept or reject it. This service is free. Your server needs internet connectivity to use it.
Anyone may view the current state of the mail queue.
Anyone may request sendmail to empty its queue.