Does your vps provider block outbound smtp to port 465/587? Usually providers only block port 25 outbound so that the vps can't send mail directly to a server (and can't host incoming mail). I haven't seen many providers block smtp altogether.

If that's the case though, services like sendgrid do offer http apis. I'm not sure if there's any sort of smtp-to-http relay bridge, but I wouldn't be surprised if there is one. Otherwise Lemmy would have to support the specific api to send e-mails through.