How to use Gmail "send as" routing vmail through virtualmin

I’ve been using Gmail’s “send mail as” feature for years but now they require you to use your own external server (Since last 2014 apparently) when setting up a new address.

My mx records for inbound are setup and forwarding to my gmail account just fine, now I need to get the outbound working.

Has anybody successfully set this up? Which ports do I use? Can I just use 25? I’ don’t really want to go to the expense of buying an SSL certificate.

Any and all help greatly appreciated…