In addition to giving an easy hit of accomplishment, email feels urgent, especially when it comes with its own sound effects--ding! This makes it an unfair competitor against long-term projects that don't shout "deal with me now."
But beyond all this, if it weren't enough that it takes time from meaningful work, email is like a drug in other ways. In addition to giving fleeting highs, email physically harms. In a piece in Huffington Post, Linda Stone coined the term "email apnea" to describe a digital age health risk (http://www.huffingtonpost.com/linda-stone/just-breathe-building-the_b_85651.html). When checking email, we often stop breathing, unconsciously holding the breath or taking shallow breaths. This suggests to me both the psychic anxiety that email checking can cause, as well as the health risks associated.
As for solutions, it seems like quarantining email as much as possible might be a good start. When we have to check email, it might be good to invoke the conscious breathing learned in yoga class--just breathe through that inbox!