I found something quite bizarre. I am using Zimbra ZCS as Home Lan Mail Server and it seems to work great. I am able to (automatically) retrieve external POP into my Zimbra Server without any issue... And I am also able to DELETE external mail once retrieved into ZCS. With any POP Connection (using also non standard ports) BUT NOT WITH GMAIL accounts: I mean: I am able to retrieve gMail mails through POP, but not able to DELETE them... even if I selected the DELETE MAIL option in the External Account setup window.
So in order to delete mail sitting in gMail account, I have to log in in gMail and delete them manually...
Any particolar reason for that? Is it a gMail issue or a Zimbra Issue