One potential "switcher" in my organization mentioned his dream feature: mail notification via RSS. I checked the Zimbra docs and here it is.

I have been experimented a bit and it works fine.

I have encountered the following problem when asking for a tag that does not exist. I guess it is an exception that is not handled properly.

Code:
type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

javax.servlet.ServletException: no such tag: GAA
	com.zimbra.cs.service.ZimbraBasicAuthServlet.doGet(ZimbraBasicAuthServlet.java:110)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
	com.zimbra.cs.servlet.ZimbraServlet.service(ZimbraServlet.java:133)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

root cause

com.zimbra.cs.mailbox.MailServiceException$NoSuchItemException: no such tag: GAA
	com.zimbra.cs.mailbox.MailServiceException.NO_SUCH_TAG(MailServiceException.java:162)
	com.zimbra.cs.mailbox.Mailbox.getTagByName(Mailbox.java:1687)
	com.zimbra.cs.index.queryparser.ZimbraQueryParser.GetQuery(ZimbraQueryParser.java:176)
	com.zimbra.cs.index.queryparser.ZimbraQueryParser.Rhs_Text(ZimbraQueryParser.java:366)
	com.zimbra.cs.index.queryparser.ZimbraQueryParser.Clause(ZimbraQueryParser.java:716)
	com.zimbra.cs.index.queryparser.ZimbraQueryParser.Query(ZimbraQueryParser.java:818)
	com.zimbra.cs.index.queryparser.ZimbraQueryParser.DoParse(ZimbraQueryParser.java:886)
	com.zimbra.cs.index.queryparser.ZimbraQueryParser.Parse(ZimbraQueryParser.java:233)
	com.zimbra.cs.index.ZimbraQuery.<init>(ZimbraQuery.java:1441)
	com.zimbra.cs.mailbox.Mailbox.search(Mailbox.java:1917)
	com.zimbra.cs.mailbox.Mailbox.search(Mailbox.java:1907)
	com.zimbra.cs.service.RssServlet.doAuthGet(RssServlet.java:91)
	com.zimbra.cs.service.ZimbraBasicAuthServlet.doGet(ZimbraBasicAuthServlet.java:108)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
	com.zimbra.cs.servlet.ZimbraServlet.service(ZimbraServlet.java:133)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

note The full stack trace of the root cause is available in the Apache Tomcat/5.5.7 logs.