Email verification for signup and adding sender's details

# Issue

There is no feature currently available to verify if user is signing up from a valid email id or not.
same issue when adding senders details

# Requirement

Need to add a feature to send mails to verify email id used for signing up.
Need to send mails to verify the emails setup for sending mails

The issue names no files, routes, or tests; start by tracing signup and sender-details flows in the React/Express/Node.js application and inspect the existing Google OAuth and mail-sending paths. Define verification behavior for both email uses, then add coverage for valid, invalid, and unverified addresses; done means both flows verify the relevant email before use.

Written by the indexing model from the issue text.