The most you can do to opt-out of “Microsoft Viva” emails is to go to

https://cortana.office.com/

and to switch off “Cortana Briefing”.

This is an invasive, unnecessary, never requested feature, that harvests your data – namely emails – for situations like appointments, compromises assumed with other people, etc.

Odds are that you will stop receiving the “Viva” emails, but your data will remain training the Cortana agent, which is unfortunate.