Looking at the dump, the "plain" password doesn't look plain to me. If it was plain it would be 'plaintext_pw_here'.
Wrong. PLAIN authentication requires the username and password to be sent as a NULL-separated string encoded in base64. You can get a valid authentication string like this by running the following on the command line.
perl -MMIME::Base64 -e 'print encode_base64("\000localpart\@domain.com\000password")'
You probably want to run the auth_types() function from Net::SMTP_auth to find out exactly what values you can use for the auth type when interacting with that server
No need for that either, the dump output clearly tells him:
250-AUTH LOGIN PLAIN XYMCOOKIE
says he can either use the LOGIN, PLAIN or XYMCOOKIE methods
The problem he is having is most probably due to the fact that he says he needs to use SSL via port 465, but isn't actually doing that(as gman already correctly pointed out).