Email Addresses
Valid email addresses are effectively an evolving standard.
Contents
Structure
An email address is composed of a local part, an at sign (@), and a domain part.
Local Part
The local part of an email address is fundamentally a name composed of letters, numbers, dashes (-), pluses (+), single quotes ('), and periods (.). The first and final characters typically need to be alphanumeric. A non-compliant local part needs to be quoted with double-quotes (").
A mail server should, upon receiving mail, either relay it elsewhere unchanged or deliver it locally. This decision should be made based on the domain part of the mail recipient; the server should take ownership over it's own (sub)domain and leave all other mail to other servers.
But the reality is that a mail server has full control over the deliverability of locally-destined mail, so effectively anything that a mail server supports is an allowed local part. Many mail servers have decided upon non-standard rules for what is valid and what is invalid. Beyond that, many mail servers do not preserve relayed mail; addresses can be case-folded and re-encoded.
TODO: copy table of service providers' support for unusual local parts from https://www.youtube.com/watch?v=mrGfahzt-4Q
Domain Part
Anything that is supported by your operating system's name resolution.