You offer your enterprise software for $5 (renewable!) for a couple of days
Your website is so poor it is hard-coded to reject valid email addresses
You’ve added an error message that explains you “are sorry” for the fact that it’s rejecting valid email addresses, and that you intend to “fix this soon”.
You’ve got an extra licensee – but you don’t have my email address. You now have, in your database, a spam address on a famous spam-eating website, one that *no human will ever read*.
Congratulations: You just gave away free value, but lost the option to send me any emails in the future. And made sure that my very first impression of you – before I’ve even tried your product support – is that you’re, well, a bit incompetent when it comes to web and internet technology. That’s not a great start considering your only products are … expensive, licensed, web and internet technologies.
That’s Web 0.1 :).
EDIT: for clarification, the problem was that my email address contained a plus symbol.