If your provider allows you to, you can send yourself a mail with the
EICAR test file. Avast should respond to that.
If your provider doesn't allow you to send "infected" mails, and you have Avast checking outgoing mail, Avast should detect you are trying to send a "infected" mail.
Don't worry. The eicar test file is harmless, it only tests your av which in this case is Avast.
You can also check the email settings in your mail client. The settings of your pop and smtp server should look different now from what you had put in there.