If it has not been too long since the message was deleted you can simply log-in (if you aren't already) to your Yahoo Client account on the Internet and find the message in the "Trash" folder of your Yahoo account (keep in mind that at some point Yahoo will remove Deleted "Trash" folder messages).
If you do not find the message in the Yahoo accounts "Trash" folder you may be able to recover the message using this Yahoo link:
Recover lost or deleted emails
Btw, for overall Yahoo Help, here is the Help for Yahoo Account website with the "Mail" tab page selected for that website.
POP Peeper: Tech support, suggestions, discussion, etc.
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