Hi Richard. Thers a good chance you have mail preferences setup to compose new emails in "plain text". And/Or, "Use the same message format as the original message", meaning if you reply to someone who sent you an email that is in Rich Text Format, then your reply will be in Rich Text Format but, if they send you an email in Plain Text Format, then your reply, and possibly any forwarding will be sent in Plain Text also.
For website addresses to be "clickable links" in mail, you need to ensure all emails you send, forward, or reply are in Rich Text Format.
go to Mail... Preferences... then select the Composing tab and ensure Message Format is set to "Rich Text".
Further below in the same tab you will see "Responding" and a tick box for Use the same message format as the original message (Respond using Rich or Plain Text).
If you always want to respond or forward in Rich Text only and every time, then deselect that option.
All links should work and be clickable from then onwards.
You can highlight a non working website link in the email body, right click, then select "Link" then "Add Link" to make it clickable but, you will receive a notification about converting this message to rich text format?"
Basically, the links will not be clickable if you are setup to compose new emails in Plain Text. Nor will any replies or forwards you send that are changing to Plain Text to match the format received by you from others.
Always end, reply and forward in Rich Text Format
Hope that helps.