In format=flowed mails created with Apple Mail EF does not correctly recognize long URLs. To follow a link I have to open the message in Mail.app (which is easy)
I think this is due to a bug in Mail. According to the format=flowed specification, it should not be inserting whitespace in the middle of a long URL:
A generating agent SHOULD NOT insert white space into a word (a sequence of printable characters not containing spaces). If faced with a word which exceeds 79 characters (but less than 998 characters, the [SMTP] limit on line length), the agent SHOULD send the word as is and exceed the 79-character limit on line length.
That said, I’ll see if I can do something to ameliorate this problem.