Activity emails aren't sent when using sender's primary email address
Are your volunteers are using an email from an external provider as their main address?

If it's the case, say john.doe@yahoo.com or john.doe@gmail.com, it's "normal" that the emails are automatically filtered and flagged as spam:
It's a feature for pretty much any big mail provider to whitelist what servers are authorised to send emails on their behalf (SPF) in order to limit spam and prevent "rogue" servers to try to impersonate one of their user.

Obviously, your civicrm server isn't one of the authorised servers, so it ends up directly into the spam, or even simply discarded.

The way emails are sent with civimail is different (no matter what you put as the from sender, it's the email that you defined as bounce that is used to really send the email) so it might be why you experience something different there

Try to use as the sender for "regular" emails an email from a domain you control (your own), the email should be sent properly