Crikey error when user fill not existing email

Form plugin working well, until user make mistake in his email field. When he fills not existing email, he gets Crikey! error:
Expected response code 250 but got code "550", with message "550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup "

mailer engine is SMTP, direct to our Exchange server.

Any idea how to avoid this?