Why do bitcoin: URLs not use "//", breaking "clickability"? I want to have something like this in a plaintext e-mail:
Pay directly: bitcoin:addresshere?amount=0.001&amp;label=Blablabla&amp;message=Blablabla

But if I do that, it won't be &quot;clickable&quot;, because the standard URL format would be:
Pay directly: bitcoin://addresshere?amount=0.001&amp;label=Blablabla&amp;message=Blablabla

Why does the bitcoin: URL scheme not use the // part? Is it intentional to not be clickable? Or am I wrong in my assumption that e-mail clients will not make it &quot;clickable&quot;?
The Bitcoin URI scheme is defined in BIP21:

Bitcoin URIs follow the general format for URIs as set forth in RFC
3986. The path component consists of a bitcoin address, and the query component provides additional payment options.

You can read through the entire RFC document or the summary on Wikipedia which answers your question:

An optional authority component preceded by two slashes (//),
comprising:

An optional userinfo subcomponent that may consist of a user name and an optional password preceded by a colon (:), followed by an at
symbol (@). Use of the format username:password in the userinfo
subcomponent is deprecated for security reasons. Applications should
not render as clear text any data after the first colon (:) found
within a userinfo subcomponent unless the data after the colon is the
empty string (indicating no password).
A host subcomponent, consisting of either a registered name (including but not limited to a hostname), or an IP address. IPv4
addresses must be in dot-decimal notation, and IPv6 addresses must be
enclosed in brackets ([]).[24][c]
An optional port subcomponent preceded by a colon (:).


Conclusion: the Bitcoin URI scheme is RFC-compliant and does not require the optional &quot;authority&quot; component which is what the // deliniates.