Is there an unambiguous syntax for URLs in a mail item?

I have a situation where I am creating an email that gets piped to
sendmail for delivery.  Everything works fine, except:

The email contains a (long-ish) URL and I find that when it reaches it
destination, sometimes the URL is rendered as a link (thus clickable) and
sometimes not.  It depends on which client the user uses to access the
email.  With Thunderbird, it works correctly, but when accessed via an
email web interface, the URL just displays as text (thus not clickable).

I seem to remember that there is a way to "wrap" a URL in an email, so that
it will always be seen as a URL, but can't remember how at the moment.
Something like how you can wrap an email address in <> and that protects it.

Can anyone help with this?

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