Or you could make sure vendors follow RFC2822. Now admittedly its dense to read, but the two sections that are relevant are; https://tools.ietf.org/html/rfc28

Or you could make sure vendors follow RFC2822. Now admittedly its dense to read, but the two sections that are relevant are; https://tools.ietf.org/html/rfc2822#section-3.2.4 https://tools.ietf.org/html/rfc2822#section-3.2.4 & https://tools.ietf.org/html/rfc2822#section-3.4.1 https://tools.ietf.org/html/rfc2822#section-3.4.1 3.2.4 explains what a dot-atom can be made up of 3.4.1 explains what can be accepted in the make up of an address. I'll always prefer vendors operating within spec even if the spec is a dense as fuck.