Mail account won't connect (but works in other client)

Since December 2015, my @videotron.qc.ca email address does not work anymore in Mail. I spoke with three techs at Videotron and they can't understand the reason of that connection problem. The account settings are all good. The password has been updated and checked. The username, outgoing and ingoing servers, port, etc... everything is good.

The connection works in another mail client such as Thundebird 😟 but not in Mail.

Email domain: videotron.qc.ca

OS version: OS X 10.11 El Capitan

The people at Videotron say that Apple servers ar interfering and are blocking my account in some unknown way. Is that possible?