(Excerpt: Paul Dyke - http://www.adestra.com/what-need-know-using-fonts-in-email/)
em doesn’t work with fonts in email as email clients have different default sizes.
% is another scalable font unit, similar to em except 100% will always equal the default size.
pt is sometimes used. One point is equal to 1/72 of an inch. But why use a unit that is a throwback to the printing industry?
In my opinion, pixels are the best option. Email designs are often pixel perfect – lots of sliced images in different table cells that have to appear seamless. Having your text behave in a similar way makes sense.
And it's also my opinion