Here's the thing, Intuit: that *is* a valid email address. Gmail has this really neat feature where you can append "+[foo]" to your email handle, and it'll still get delivered to your regular email address.
Why would I want to do this? It's a great way to filter email, for one. You can simply set up a filter that searches for the "+" strings, dropping them neatly into different folders.
More importantly, using the "+" is a great way to figure out who's selling your email address to spammers. If all of a sudden you find that you're receiving lots more spam to your "+intuit" address, you can be sure that Intuit has sold your address to spammers and other ne'r-do-wells.
So is that what companies are afraid of? Are they afraid that they'll be found out as a company that aids and abets spammers?
Or is it simply that their programmers are too lazy to write accurate validation code for their web forms?
Caveat: I'm not calling out Intuit exclusively on this. Intuit's a great company and I personally use several of their products. And this has happened to me at many other sites.
Maybe I should start my own little "accept the plus sign" Internet meme.