[CentOS] E-mail advice sought

Sat Apr 30 18:28:23 UTC 2016
Alice Wonder <alice at domblogger.net>

I'm working on setting up an e-mail service.

I've got the e-mail servers working beautifully and am presently working 
on re-writing the parts of Roundcube I don't like (e.g. it uses inline 
JavaScript in a few places so CSP breaks it) but -

Is there any advice on characters to allow in usernames?

I know there are some wacky characters that are legal in e-mail 
addresses but are generally frowned upon - like

"very.(),:;<>[]\".VERY.\"very@\ \"very\".unusual"@example.com

is apparently a legal address - but I know I don't want to allow 
ampersands and brackets etc. in an address.

I don't think a whitelist alphabet is best approach because of people 
with names that are not spelled with Latin characters.

Is there an existing blacklist of characters that technically legal but 
are generally avoided in e-mail addresses?