I'm not at the moment but a couple of reasons that I experienced issues with links in an email in the past include:
Misplacement of the 'http://' in a token for example if you place the http outside of the token and inside the token itself so that url technically has a duplicate http like this - 'http://http://www.linktowebsite.com'
Switching to a dedicated IP address and IT hasn't properly updated the existing security settings
Changes to existing firewall settings
Hope this helps.
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