mail to sympatico

strange issue. On my Mac (and my wife's) running latest software (Venture 13.5) sending from Apple mail client my emails are not being sent. I used Bell Canada sympatico. Was working fine up to a fw days ago (I think). I've verified my settings all good. I deleted and readded the account. Still nothing. rebooted my modem. Nothing. But sending works from Thunderbird, and other email clients (Outlook, Edison). Interestingly, from my iphone both on wifi and off wifi, it works as well. I also tried firing up my VPN (I[m in Canada so I connected to Montreal) still nothing on the MAC. I am baffled. The outgoing server hs 2 ports 587 or 25. tried both. Nothing being sent. I am completely baffled. Any thoughts?