To prevent spam from filtering into your Lead Dropbox, only emails sent from "Trusted" email addresses will be accepted.
You can accept all emails from a particular domain by adding a * as the name part of the address e.g. "*@somedomain.com" would allow you to receive emails from "email@example.com", "firstname.lastname@example.org" etc.
If you set an enquiry source for your Trusted Email, this enquiry source will be automatically applied to any emails received from the given domain.
All trusted addresses here apply to the account / "unassigned" Lead Dropbox email listed here but also extend to all individual user Lead Dropboxes. This means that if you add an email address here, user's will also be able to receive emails into their personal Lead Dropboxes from the address you add.
Please visit the Leads help articles for additional information.