This has to do with the encoding of an email when it is outbound and the particular client used to view it. RFC 2045 states that the standard MIME encoding line length maximum is 76. Many email encoders will change this which sometimes causes corruption.

Have you checked the message source to see if it is correct as it comes out of the SMTP server to the recipient? If it is broken perhaps some changes need to be made to the mail code or php mail() settings.

Otherwise, it is a client issue in that it is splitting lines after a certain length. There isn't really a solution to that problem that I have to offer. I would change the template to include the text of the link with copy like "If the below link doesn't work copy and past it into another browser window". That is pretty standard practice in the email game.

Also a total aside, I've had to trace problems on both sides of email sending before. You would be amazed what kind of errors Gmail will find, identify, and fix that are totally transparent to users. I can only imagine that other web based and standalone clients do.