Look up user email from user name

[Jonathan Rosenberg]

I am using Automation for Jira to send a custom assignee change email, so I need the new assignee email address and the previous assignee email address. 

Using the changeField smart value I can get the previous assignee username as a string, but I can't find an easy way to return the email address from the username string.

If it can't be done in Automation, I would have to write a listener from scratch in ScriptRunner.

Thanks!