Did this issue just occur out of the blue? or is this a new email draft?
Are all your outgoing emails links being prefixed with:
Or just this one email? Could the above be hard coded into the template?
I'm not seeing it hardcoded in, and I'm not seeing the issue with any other links on any other emails. It seems specific to this one.
Here's the solution I found after much digging. These emails are built using Marketo templates, and Marketo was adding curly quotes which were visible in the code, but weren't visible in the GUI.
So the link in the raw Marketo code should have looked like this: href="${ctaLink}"
But because of an apparent bug in Marketo the link looked like this: href="“${ctaLink}""
Any link which had the additional quotation marks was broken, but once those were removed the links work perfectly.