I contacted Arvixe support, they're telling me to use port 25 and localhost as host. Tried both settings (secure true and false on both key="EnableSmtpSSL" and enableSsl="true"), but still get the same error. Now I tried the same stuff with gmail services, both secure and not, on ports 587 (throws the error instantly) and 465(times out)...
Quote from Arvixe support reply: "Have you enabled smtp authentication in your script? Please contact a developer and make sure SMTP authentication is enabled in your script." :)