Is a mirror link mandatory in any email?

I have to design emails on mobile first for B2B users. Some professional mailboxes don’t support HTML5, and display may be broken. I wonder if the mirror link (e.g. ‘If you can’t see this email, click here’) is mandatory in any email? Is this a good practice? Or can we proceed differently to ensure the access to the content of email?

Thanks in advance for your feedback