Imap_rfc822_parse_adrlist is too strict: if a double ,, is present in addresslist, then the parsing fails and stops (and gives various SYNTAX-ERROR/INVALID HOST)
This is really annoying as many users type double ,, or even , , (a comma, a space, a comma) causing most of recipients to be dropped if Imap_rfc822_parse_adrlist is used.

Example:
imap_rfc822_parse_adrlist("smith,jeff,paul","foobar.com")
WORKS
  -BUT-
imap_rfc822_parse_adrlist("smith,,jeff,paul","foobar.com")
DOES NOT WORK: only "smith" is extracted, BUT jeff and paul aren't

Workaround: while exploding recipient list, just IGNORE empty recipient as if they didn't exist (and of course continue parsing)