Hi,
It would be wonderful if Marketo had automated rules set up to identify and label email addresses as invalid when they don't meet certain criteria (ie - there's no @ included in the data that's in the cell).
Right now, you can create your own smart list to look for common errors in emails (ie - no @ symbol; ".cm" rather than ".com", etc) but I see no reason why this can't/shouldn't be an automated process that Marketo takes care of.
Thanks!