BuddyPress.org

This one can drive a person absolutely mad trying to figure out. Depending on what your setup is (Multisite-enabled?), you can try this method;

1. Create a noreply@yourdomain.com (or whatever you want to call it – you can even use a Gmail address for this, though I’ve not tried that approach myself) email address, if you don’t already have one, with SMTP relays, especially if your ISP has restrictions in place for sending email.
2. Add and activate the WP-Mail-SMTP plugin; https://wordpress.org/extend/plugins/wp-mail-smtp/
3. Configure the plugin settings to fit your needs, or as per the plugin instructions suggest, by going to “Settings” -> “Email” in your WordPress admin area.

At least that’s what I recall from my past experience with BuddyPress enabled and having this issue, and it worked like a charm!