Yahoo Mail begins blocking users with ad block enabled

I can understand where Yahoo is coming from, but doing this will divert even more users away from its already struggling mail application.

What shall they do with people who don't see ads because they connect through IMAP with a traditional email client? I rarely use the web client. Does that mean I'm stealing from them? If they offer the service for free through third-party clients, then what difference does it make I block ads in the web client?

That's just what they need to get another bunch of users to switch to Gmail.

This kind of situation is a good reason to decouple your email address from your email provider. If you use your own domain, you aren't dependent on any particular provider. It's unfortunate that the process of obtaining and using a custom domain for email is a bit too technically challenging for the average email user.

I think most would be willing to pay the perhaps $40/year for such an important service to avoid distracting ads and having their email scanned for saleable information.

How many use their web interface instead of an external client?

This is just the beginning. People think adblockers can't be defeated. The reality is that they can easily be defeated.