When composing a message in HTML mode I can copy but I can't paste.
I was trying to reuse information from a previous email created with Zimbra desktop but could not.
Also, I tried to copy a URL and text from firefox to paste and it would not work.
Copy and paste works it seems in the address and subject lines.
For some reason you can copy and paste within the new email message, that is anything you type in can be copy and pasted to any part of the the email.