The world’s largest free email provider is simply unreliable. Sure, most of the user experience is pleasant, but Yahoo bans entire network blocks when it finds a few instances of abuse. Senders receive error messages like this:

421 Message temporarily deferred

Temporary error from SMTP partner: smtp;421 Message temporarily deferred – 4.16.51. Please refer to http://help.yahoo.com/help/us/mail/defer/defer-06.html.

1. the message you attempted to send exhibited characteristics indicative of spam,and/or
2. emails from your network have been generating complaints from Yahoo! Mail users.

The problem with blocking a whole network is clear–blanket banishment harms innocent users. Do not use Yahoo mail for important messages because innocent people out there are being blocked by Yahoo. A few vandals can cause Yahoo to temporarily ban an entire ISP user base.