The invention provides, in some aspects, digital data processing methods of generating digital content pieces (e.g., email messages or portions thereof) that are customized in accord with individual recipient behaviors. Such methods include the step of generating and digitally transmitting to a digital data devices of a recipient a digital content piece that (i) has a call to action to which the recipient can respond and (ii) that has a plurality of features selected so as to maximize a probability, P(b1, b2, . . . , bM, x1, x2, . . . , xM), that the recipient will respond to that call to action, where that probability is defined by the relation P(b1,b2, . . . , bM,x1,x2, . . . , xM)=exp(Σj=1, . . . , Mbjxj)/(1+exp(Σj=1, . . . , Mbjxj)) where x1, x2, . . . , xM are values for each of a plurality, M, of features characterizing the digital content piece and/or the recipient, b1, b2, . . . , bM are respective coefficients for each of the values x1, x2, . . . , xM.