In e-mail, a probe message is a type of
bounce
handling for
e-mail address verification. It is
commonly used in subscription-based
mailing lists
that easily become filled with invalid subscriber addresses. Once an e-mail has
bounced from a specific address, a system will wait a specific amount of time
then sends a warning message to the user. If the warning message bounces a probe
message is then sent. If this also bounces the subscriber is then removed from
the mailing list.
Also see
Why
E-Mails Bounce in the
Did You Know
section of Webopedia.