Two quick points

For instructions on how to access headers from various e-mail clients/online providers, you can see

Regarding reverse IP searches - many domains it shows multiple others. I have no idea who all sites those are, because web host provider is the one being shown.

Thanks. Good link. I have added this as a reference link.

This is common if you use a hosting service. There will be many virtual servers running on it and the IP will have been associated with different domains over the lifetime of the IP address. Sometimes the information can also just be plain wrong. But a host I use for some web services my have about 10 sharing the IP. So many hundreds is unusual. Probably about cost.

You can check here too

To avoid this you need a dedicated IP or server. We have many services with AWS, digital ocean and turnkey Linux which are dedicated IPs.