You left out one very important way of covering your email. Both Outlook and Lotus Notes (server based) allow you delegate your email within the program. So your admin or the person who is covering for you will have access to your email box from within their own email client. Any email sent will be noted that it was sent by X on behalf of Y. This is controlled by the program and meets corporate governance standards.
Additionally, the recommendation that you share your login with another employee may be against company policy. Many large companies that I have worked for have policies specifically forbidding the sharing of passwords. My user ID in my company allows me to process and approve expense reports, authorize purchases, approve time-sheets, and the like, to which it would not be appropriate for a subordinate to have access.
It wouldn't be a question of whether or not, I trusted my employee, but a blanket rule to make sure our audit trails are 100% accurate.