Emails sent from Gmail not reaching my ISP email address

That's on the provider. If Outlook and Yahoo get through and only Gmail doesn't, their mail server is rejecting or silently dropping Gmail traffic, there's nothing in your settings that filters by sender's provider like that. One useful thing before you call them: have the sender check their Gmail for a bounce (from Mail Delivery Subsystem). If there is one, the error code in it is exactly what Optimum's support needs. If there's no bounce at all, tell them it's being accepted and then dropped.