Add Email length validation

[itsksaurabh]

Issue Summary

The technical limit for an email address is 64 characters before the @ sign and 255 after, allowing for a total length of 320 characters (including the @ sign).

According to https://tools.ietf.org/html/rfc3696#section-3

In addition to restrictions on syntax, there is a length limit on
   email addresses.  That limit is a maximum of 64 characters (octets)
   in the "local part" (before the "@") and a maximum of 255 characters
   (octets) in the domain part (after the "@") for a total length of 320
   characters.  Systems that handle email should be prepared to process
   addresses which are that long, even though they are rarely
   encountered

According to Sendgrid's best practices described in this blog https://sendgrid.com/blog/email-marketing-length-best-practices"

Technical details:

• sendgrid-go version: v3.8.0
• go version: 1.16