It should be 254 (http://stackoverflow.com/questions/386294/what-is-the-maximum-length-of-a-valid-email-address), but that seems improbable.. so I would go with 50 chars max, instead of 20 that cuts 4 out of 10 emails I receive.
* Adjusted mmcfe-ng occurences in code * Adjusted Database strucutre to only supply the full structure * Adjusted default template to MPOS Addresses #643