The behavior you describe sounds like you went to My Email Preferences and clicked "Edit" next to your @yahoo address, then added the dotted address as an Alternate Posting Address. Those are always converted to the main address when posted to the group.
The key step:
Go to the "My Groups" page, "Edit" tab. Use the drop-lists to select the dotted address for use in any groups you desire. The dotted address won't be shown as your "From" address in email postings unless it is selected here.
In an earlier message I think you indicated that the dotted address isn't in the drop lists.
Try this (but first remove the dotted address from the Alternate Posting Addresses if you added it there). Caveat: I haven't actually tried this with a dotted address, but this is how it works for non-Yahoo addresses.
From "My Account" (after confirming your password) click "Edit" on the "Email Information" bar. On the "Manage your email addresses" page* click "Add another" (at the bottom of the list). Add your dotted address here. There's some chance you'll get stuck here because normally you can't add an @yahoo address as an alternate to a different Yahoo account; but maybe dotted addresses are "special" in this regard.
You'll be prompted to verify the address. Yahoo will send an email to the dotted address, containing instructions to verify that you receive mail at that address (reply or enter a code at a web page).
You may want to try posting in one of the "groups about groups", someone there probably knows about dotted Yahoo addresses.
Moderator, GroupManagersForum & yahoo_group_of_groups (Yahoo Groups about Groups)
* This page is also supposed to work from "My Email Preferences", but I've run into trouble recently using it from there.
At 04:58 PM 9/7/2008, you wrote:
>I went to one to a group that has had very little traffic
>recently and sent a test email using the address with the dot in
>it. YG accepted and posted the message. The only thing is the
>dot address did not show as the sender it showed my main Yahoo
>email address. I sent the mail from Yahoo Mail and not the Mac
>Mail.app or Thunderbird. I wanted my dot address to show and NOT
>my main address. So, I guess I will not be using Yahoo for mail
>after all. I'll stick with Mailplane and Gmail(or Fastmail.fm).