Are you sure this dovecot instance allows for plaintext passwords? Check the dovecot configuration file for the for the setting

if it's set to only allow other authentication mechanisms (CRAM-MD5 or DIGEST-MD5)you'll have to use those. Also take a look at the logs if you have access, dovecot is usually rather helpful in what it tells you about errors.

Other than that, it may be helpful to look at the network traffic Evolution generates when connecting to the server successfully, as well as the traffic when your script connects. Here's a node I wrote in response to a different question, explaining some ways of how to do this.