fix(#1199): respect custom mailserver on startup

[bbortt]

closes #1199.

@e17gpy I wen't for this implementation and will skip this part (as reported):

As the Citrus MailServerBuilder does not have functions to set the host manually, and the MailServer initialises the GreenMail server with a hardcoded host value of null, the Citrus MailServer will only bind to the loopback interface and not receive Mails from another interfaces.

I think for security reasons this is ok. whoever want's to bind to a specific interface should create a custom MailServer and use that - with the provided fix this is now possible. does this fit your needs?

[ghost]

Hi, this should fit our needs. Thanks for implementing.