When sending out email messages with Tumbl links in them, many of these messages are being regarded as SPAM or being marked as bulk. As a result many of these messages are not reaching the intended recipient's inbox.
Gmail is bulking based on presence of Tumblr links in message body. Tumblr.com is listed on the Spamhaus DBL and several other URI-based dnsbls. The DBL is a list of domains that are known to be tied to spam. In Tumblr's case, they are frequently used as a redirect to a spammer's website. Tumblr has a history of not disabling the links, which has led to their DBL listing. There is potential impact in that any mailbox provider checking the DBL or any of the other lists on which Tumblr is listed is more likely to bulk or block based on the presence of tumblr.com in the email. We recommend that customers remove any tumblr.com links. Alternately, you can use your own tracking links instead of company.tumblr.com but some mailbox providers will check the final landing page (tumblr.com) and block or bulk based on that.