While % character is RFC compliant for SMTP, is there any risk in removing it from the "Illegal Mailbox Characters" list?

Although it is possible to use % in the local part of an email address because it is RFC compliant, it might be interpreted specially by some email systems.

For example % may be a valid character in a Lotus Notes, but not valid in Gmail, Yahoo, Hotmail, etc. That is why it is recommended to have at least these three characters (@!%) defined as illegal mailbox characters.