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I was wondering if there are any free, low-cost is okay too, email script/service where I can run a gmail or mailinator like service? For gmail, like a registration page.
If you want to be really cheap and nasty, you could use this with a cPanel account.
Otherwise, b1gmail or Hivemail are your best bets, although Hivemail hasn't been updated for about a year.
Need to reach me quickly? Ping me on Discord
b1gmail was great software some years ago when we used it. Have not touched it for 3-4 years though, so that might have changed. Be however aware, that receiving any support (even when it's a bug, so not actually a problem on your side) is hard.
ZImbra isn't a script, but perhaps a good solution.