This is (I think) a new development in email deliverability – with phishing scams on the rise – that could be affecting your deliverability. In particular, when you place a URL in your email with the text matching the url (e.g. http://www.website.com is the link, and http://www.website.com is also the link text), your email marketing software adds a tracking code (usually something garbled like rs6.net/234swefkjads) and it looks to email clients like a mismatch and you are potentially marked as a “phisher” and your emails could start being blocked and undelivered.

Combat this by NOT using the link URL as the link text.. (eg: instead use “Visit my site”)

Very interesting, and for more tips to ensure your emails aren’t being confused with phishing scams, read the full article by Stefan Pollard over at ClickZ