Just had this happen today to a customer. In addition to the <> value the problem can occur with the following characters in an email address:
! (although an allowed character for UUCP Bang-Path & Hybrid Addresses under RFC-822, 2822 and 5322 RegEx treats a bang similar to a *)
: and :: (although an allowed character for Route-Addr Address and DEC/FidoNet Addresses under RFC-822, 2822 and 5322 RegEx treats a colon as a non-capturing group)
% (although "Ye Olde Arpanet Kludge" addresses have not been RFC compliant for over 30 years almost every MTA still supports them...not sure if Smartermail does or not)
I would assume that the other 12 RegEx reserved characters in an email address would result in similar symptoms:
Hope that helps narrow down searches when trying to find the needle in the Blocked Senders Filter haystack.
Which brings us to a quandary...perhaps SmarterMail should be sanitizing email addresses prior to adding them to the Blocked Senders Filter to prevent addresses containing special characters from filtering all incoming email. Maybe this should be turned into a request thread.