I never had a yahoo mail account and I had no idea it is problematic. I assume those who say so are referring to webmail and free accounts and (questionable) advertising. Which anyway one can deal with. Most unwanted garbage (ie the sort that arrives on the screen without a user looking for it) is removed by using a hosts file, such as mvps (you can google mvps hosts).
Perhaps we all should realize that jumping to conclusions does not necessarily bring us to the correct conclusions. Even though k9 is quite good (but requires powerful hardware and is not very suitable for budget machines) it has false positives, even in categories that one would think are well-defined. Monitoring software can have false positives as well: an example? Someone beyond suspicion had in their log a connection to a well-known website we do not use (I do not mean FB or twitter or yahoo). Turns out someone had ran on the machine a utility for choosing a fast DNS. That utility measures the times it takes for various public DNSs to resolve a list of “most frequently used websites”, including some of those we don’t use. To do so, of course, it connects to the domain name.