Yahoo is angering the dwindling number of people who still use its email service with a test run of a feature that locks out anyone who has an ad blocker installed.

Several Yahoo users complained that they encountered an error message when trying to open their mail account on Thursday in an AdBlock Plus forum thread first spotted by Digiday. The screen ordered them to first disable ad blockers in order to access their emails.

A Yahoo spokesperson said in a statement provided to Mashable on Friday that the blocking is only being tested on a portion of Yahoo email users.

"At Yahoo, we are continually developing and testing new product experiences," Yahoo said. "This is a test we're running for a small number of Yahoo Mail users in the U.S."

Predictably, people were not happy with the perceived strong-arming of their email accounts, and some even threatened on Twitter to leave Yahoo for good if the lockout continued.

"please disable ad blocker to continue using yahoo mail" how about no. do you want me to stop using yahoo mail?? cause this is a good start — Timmay (@RunLikeDeer) November 19, 2015

Hey @Yahoo my grandpa called from afterlife. He wants to read his mail. Stop doing this https://t.co/6TiFZPT4RD — Rutwij Devashrayee (@rutwij) November 20, 2015

So @YahooMail has blocked my inbox for using an ad blocker. It was a good run, I guess. Goodbye! Hello Apple Mail, as much as I hate it. — Andrei Herasimchuk (@Trenti) November 19, 2015

Yahoo joins a growing number of media sites to experiment with barring ad block users altogether as the technology available to do so becomes more widespread. The Washington Post tested out a feature that directed ad block users to a subscription page in September, and the London-based newspaper City AM started a similar trial this week.

New York Times CEO Mark Thompson also said in a recent earnings call that the paper is looking into "technical solutions" to combat blockers.

It's easy to see why Yahoo has an interest in thwarting ad blockers. The company posted disappointing third quarter results last month that may point to a broad decline for the company, and up until last summer, it had experienced several consecutive quarters of flagging advertising revenue.