In the project, there was a use-case where we have to use different SMTP configurations for sending mails from different user accounts. To elaborate, each user in the system can create a 'site' which sends notification mails using custom mailing configuration provided by the user. I had a look at the grails-mail plugin, but found out that it currently did not support multiple MailSenders.
So I modified the plugin and added a new method in which we could specify a custom MailSender. This blog post contains information on how I did it.