You're welcome.
acohen wrote:I'm familiar with the ability to specify whether retrieving email should delete it from the server...
That's a POP3 function.
acohen wrote:I want to be able to retain email on the server whenever routinely retrieved with my phone, but also to be able to choose to use webmail to manually delete spam when circumstances make that best, like on a weekend when I don't check my desktop and spam piles up.
Which is why we're recommending
IMAP. It processes email the same way as webmail, but works through the mail client. Among other advantages, it means that only the data about the email in your box is transferred to your phone, while webmail requires the entire screen to be transmitted to your phone, and that your web browser has to format the display. In particular, the phone doesn't "retrieve" the email; it only gets the list of messages (subject, date/time, sender, etc.), and presents the list. An individual message can be opened, but it will not be deleted from the server unless the user specifically deletes it. Just like webmail.
Perhaps you're not familiar with IMAP?
Here's a good comparison between POP3 and IMAP. (You can find other comparisons if you ask the Google "difference POP3 IMAP".) IMAP gives you the same functionality that you're seeking with webmail, except it uses the native app on Android (and other) phones. This means the display is consistent between mailboxes, it can access the contacts in your phone, and you can even choose to have a notification when mail arrives.