user113690: I want to have something like this in a plaintext e-mail:
Pay directly: bitcoin:addresshere?amount=0.001&label=Blablabla&message=Blablabla

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

user3667: 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 "authority" component which is what the // deliniates.

113690: <p>I want to have something like this in a plaintext e-mail:</p>
<pre><code>Pay directly: bitcoin:addresshere?amount=0.001&amp;label=Blablabla&amp;message=Blablabla
</code></pre>
<p>But if I do that, it won't be &quot;clickable&quot;, because the standard URL format would be:</p>
<pre><code>Pay directly: bitcoin://addresshere?amount=0.001&amp;label=Blablabla&amp;message=Blablabla
</code></pre>
<p>Why does the <code>bitcoin:</code> URL scheme not use the <code>//</code> 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;?</p>
 3667: <p>The Bitcoin URI scheme is defined in <a href="https://github.com/bitcoin/bips/blob/master/bip-0021.mediawiki" rel="nofollow noreferrer">BIP21</a>:</p>
<blockquote>
<p>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.</p>
</blockquote>
<p>You can read through <a href="https://www.rfc-editor.org/rfc/rfc3986" rel="nofollow noreferrer">the entire RFC document</a> or the summary on <a href="https://en.wikipedia.org/wiki/Uniform_Resource_Identifier#Syntax" rel="nofollow noreferrer">Wikipedia</a> which answers your question:</p>
<blockquote>
<p>An optional authority component preceded by two slashes (//),
comprising:</p>
<ul>
<li>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).</li>
<li>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]</li>
<li>An optional port subcomponent preceded by a colon (:).</li>
</ul>
</blockquote>
<p>Conclusion: the Bitcoin URI scheme is RFC-compliant and does not require the optional &quot;authority&quot; component which is what the <code>//</code> deliniates.</p>
