In any email there are two relevant sender addresses:

Header "From:" Address

The one users are most familiar with is the header "From:" address, shown as the email's sender to message recipients.

SMTP Mail From Address

The other sender address is the SMTP Mail From address, which is sometimes called the "envelope From" address. This address is not part of the message but is sent in the SMTP conversation between email servers with the "MAIL FROM" command. Email servers use the SMTP Mail From address to determine where to return undeliverable email.

Typically, the SMTP Mail From address and the header "From" address are the same address. Sometimes, however, a bulk mailing system will use a different SMTP Mail From address.

If your server/device does this, be sure to verify that both addresses—SMTP Mail From address and the header "From" address—are registered with AppSMTP.

For information about registering addresses with the AppSMTP service, visit the main AppSMTP page.