By default, on the user id is used when dispatching email messages. Using this check box, the user name is also used. Spaces are replaced by dots. For a given 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. At some point your organisation may end with two persons named identically. You will have to rename the first account. Unless you do that, the older user will receive message for the new user. The proper solution is to use only the account name, so correspondent can't guess blindly the email address.
Relay control lets you decide who may use your SMTP server (sendmail) to relay messages. Spammers are often borrowing SMTP servers to explode a message to a huge list of people. By configuring relay control, you can limit that. Unfortunately, mis-configuring 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://mail-abuse.org to learn about this feature. For every mail received, a special server is queried to accept or reject it.
The VERIFY command allows query of an account existence from the the mail transport protocol. In general, you do not want outsider to probe your server like this.
The EXPN command expands an address. If the address corresponds to a mailing list, it will present all the members of the list. In general, you do not want outsider to probe your server like this.