Having a mirror link in your email campaign is not mandatory.
Is view this email in your browser necessary?
Reasons to have a 'View this email in your browser' option
To eliminate any concerns about display features in certain email applications. To make your email code accessible to your users. To encourage social sharing. To allow people to bookmark your email if you're sharing helpful information.
Why do my emails say view in browser?
The "view in browser" link points to the web-hosted version of your email, which is kept on our servers. This helps recipients who either can't view HTML emails, or have image downloading switched off, to view the full version of your email.