I cannot send my Emails from the Mail program

I use the Mail program of my Mac Book pro for different accounts.

After having switch it off for 3 days, I switch it on as usual and I realized that I can receive my E-mails but I cannot send any. I suppose it has something to do with the SMTP. Yesterday I followed the instructions to fix the "Mail" and it was working again but today I have the same trouble again.

Virus ???