I could wing it, but [link|http://mathworld.wolfram.com/StatisticalTest.html|Wolfram's already wung it good].

Measurement of type I and type II error are independent. You're measuring the percentages within two independent populations: spam and non-spam. The overall error would be the sum of the type I and type II errors, over the sum of all mail received.

Any way you add it up, SA rocks hard.