Q: Mail 9.3 (3124)

When I send a message in Mail, I get the following response from Mail: impossible d'envoyer le message via le serveur relais.videotron.ca

iMac 21.5 - mid 2010

3.06 GHz Intel Core i3

memory 8 Go

storage 419 Go/499 Go

OS Mac OS X El Capitan 10.11.6