You are allowed to define simple transformation rules which will be applied to all emails being forward by sendmail. Typically, you specify a from and how this from should be translated.
You have some flexibility here. You can translate from:
Here is a presentation of the different fields of a rule's dialog.
You are allowed to activate/deactivate a rule without erasing it using this checkbox.
You must enter an email address. Optionally, you may omit the user name and only supply the domain name prefixed with the '@' character. This means that the rule applies to any user of the domain.
The "from" email address will be rewritten using this field. You must again specify an email address. Again, you may omit the user name and just provide the domain name prefixed with the '@' character.
Enter a comment to remind you about the reasons you used this feature (the masquerading rule). Normally, masquerading rules are used last. It is better to fix the problem at the origin. Unfortunately, this is not always possible, for all kinds of very good (and stupid) reasons.