I would agree that customization of what is considered the "+" is necessary. The problem I'm having is that many (many) sites simple do not allow a "+" character in their emails...rendering this feature useless.
As well, the "/" is rarely accepted (used to denote folders).
My ultimate set of features would be:
1) Option to identify what the "+" symbol is. For me, it would likely be a "-" or "_", or as JP above stated, a ".".
2) Option to identify what the "/" (folder delineation) symbol is.
It should be allowed so that #1 and #2 can be the SAME symbol (if desired). So, if that symbol was a "-" then the address al-shopping-amazon@blah.com would go into a Shopping/Amazon folder.
For the time being I've been using "-" and creating aliases (PITA) when sites don't allow the "+"...and there's a lot of 'em.
Thanks for your consideration!
AL