I've had a complaint from a customer that they can't reply to our user generated email addresses - there are various reasons for the format below, but the long and short of it is that the Yahoo! Mail web interface doesn't like it and just spits out an error:
We cannot find a match for this email address:
Please use an email address that looks like firstname.lastname@example.org or a nickname or a distribution list name from your Yahoo! contacts.
Does anyone know why this isn't passing the Yahoo! Mail filter?