If you already have the $output (the part that Sanford mentioned), I'd say that you can also do something along these lines:
#set( $output = $input.replaceAll( $regex, "$1\u001e$2\u001e$3" ).split("\u001e") )
#set( $outputA = $output )
#set( $outputB = $output )
#set( $outputC = $output )
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Thanks to Markus, I was able to login to @t-online.de webmail and see the problem firsthand.
And it definitely is a T-Online problem, not a Marketo problem. Marketo is sending totally valid HTML, but the T-Online HTML parser must've been built by someone who didn't read the HTML standard (which hasn't changed in this regard since at least HTML 3.2!) carefully. And no one ever reported the bug and it's been sitting there for who-knows-how-long.
Unfortunately, I have no single answer for you. It's going to come down to how important these leads are. We discussed this very case in our MUG office hours today and here are the strategies to choose from (more than one might be used simultaneously):
Send Text-Only versions emails to T-Online users.
Using Velocity, include a more exaggerated pointer to the Web View for the T-Online crowd. You might mention specifically that you know they'll have trouble because they use T-Online, or let leave that unsaid but feature the Web View much more prominently for them.
Create a Segmentation like Known Broken Links that separates out T-Online and any other known problematic domains. Segment any part of the email that has links and don't have them be true hrefs, i.e. just include the link at the end of a sentence, like Go to <https://www.example.com> now. Email clients will render that as a link.
Send emails to T-Online users via a SMTP gateway that changes the HTML on-the-fly before forwarding to the end user. This way they'll see the links like everybody else. (This requires a skilled sysadmin-slash-developer but it's not a towering task if someone is familiar with this type of thing.)
Communicate to Sales that you have this problematic cohort — maybe they can cajole people into supplying different addresses, or subtly communicate that there may be some problems due to old webmail and to not hesitate to reach out.
Review the t-online.de leads based on other characteristics to see projected value (though this can understandably be extremely difficult).
Finally, I thought this was a pretty funny tip on T-Online's site:
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