Email providers that support @*.onion email addresses - Email Privacy - Fedia
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There are very few onion email providers. I’m hoping to crowd-source more options. These are the onion email providers I am aware of:...

As the linked post demonstrates, if you enter a link like this:

[mail2tor](mail2torjgmxgexntbrmhvgluavhj7ouul5yar6ylbvjkxwqf6ixkwyd.onion)

mbin thinks it’s something else. Indeed it’s not a URL due to the lacking ‘scheme://’, but it’s bizarre what it does with the links.

Since SSL is not generally needed for onions, every link would require some effort to know whether it should have a scheme of http:// or https://. Mbin should just pick one of those schemes arbitrarily… certainly not whatever it’s doing at the moment.