We have a few products and mailing lists so we have our own install of Sendy running.

Each product gets at least 2 lists. One for marketing and one for users (ie, a required message).

We add/remove users to lists via API integrations within the apps themselves.

May not be useful if you only care about 1 product but Sendy is so cheap, and I run it on a $5/mo VPS. Plus SES costs, It's been fine for us so far and very cheap.

Oops, just saw your addition of "in realtime", you're going to want to use Popen. See the docs.

Easiest method is using the subprocess module. If you're using Py3.5+ (you should be) see:

docs.python.org/3/library/sub…

Keepassxc on Linux, KeePassDroid on my Pixel2, db file sync'd via Dropbox. If you're talking about on your app or code, then each situation is different but it's normal to have environment variables store that or special configs that are only stored in the environments that need to use them.