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T-Mobile is famous for its attacks on rival carriers — but its latest promotion targeted poor BlackBerry, encouraging users to switch to an iPhone. Both BlackBerry users and CEO John Chen fired back, with Chen saying in a heated blog post that he is "outraged" at T-Mobile.

Late last week, T-Mobile emailed customers the offer aimed at current BlackBerry users: Switch to an iPhone 5S for $0 down.

That BlackBerry userbase may be shrinking, but as T-Mobile quickly found out, it's passionate. BlackBerry users struck back at T-Mobile for the perceived slight via social media and other forums, while BlackBerry CEO John Chen responded with a sharply worded company blog post on Tuesday.

Chen slammed T-Mobile's promotion as "clearly inappropriate and ill-conceived." He said he was "puzzled" T-Mobile didn't tell Blackberry about its plans ahead of time, and he thanked the BlackBerry faithful for bombarding T-Mobile with tweets, calls and other messages.

"[Y]ou sent a powerful message that T-Mobile could not ignore," Chen wrote. "Your partnership with our brand is appreciated by all of us at BlackBerry, and draws a sharp contrast with the behavior of our longtime business partner."

What's more, Chen promised, "for our loyal customers on the T-Mobile network, know that we have an offer in the works designed especially for you. Watch this space for an update very soon."

T-Mobile told NBC News in an emailed statement Wednesday that the company is "happy to be a BlackBerry partner and apologize for any confusion." In an apparent olive-branch effort, T-Mobile will offer free expedited shipping on BlackBerry devices starting Friday.

T-Mobile's CEO, John Legere, didn't exactly apologize for the promotion, but he sent a series of tweets earlier in the week -- before Chen's post -- in response to the BlackBerry fans who complained.

He later followed up:

Chen, meanwhile, closed his blog post with a finger-wag at T-Mobile.

"Finally, to T-Mobile, I would like to remind you that our long-standing partnership was once productive and profitable for both BlackBerry and T-Mobile," Chen wrote. "I hope we can find a way forward that allows us to serve our shared customers once again."