LAST command

I am afraid I don't understand your question. You can not use Yahoo Mail to read AOL Mail.

You can either use AOL webmail (http://webmail.aol.com) or use any desktop mail client (like Thunderbird or Outlook) and access AOL mail via the IMAP or POP3 protocol.

Maybe I can answer your question better if you explain what you want to achieve and what you mean with "LAST" command. 

I guess you're referring to the POP3 LAST command. This will download the messages (via POP3) which are new since the LAST poll. This will not download unread messages only, since the POP3 protocol does not differentiate between read and unread.

If you are pulling mails from AOL into your Yahoo Inbox via POP3 - you might want to check (wherever in Yahoo) to erase the mails from AOL once pulled via Pop3 (sometimes this options is called "leave a copy on the server". This ensures that only the mail which haven't been pulled gets pulled into your 3rd party inbox.  

However, I still recommend reading your AOL Mail via http://webmail.aol.com - or using any other mail client which talks IMAP (rather then POP3).

LAST is not part of the current POP3 standard (RFC1939) which is about 12 years old, and it was removed in the prior standard some 14 years ago. If some Yahoo's client implementation relies on it, then they're more than a bit behind in the times.

Some interesting comments on LAST are:

  http://jeremy.zawodny.com/misc/yahoomail-and-wcnet.html 

  http://www.remote.org/jochen/mail/info/pop3-last.html

  http://www.dovecot.org/list/dovecot/2004-June/003775.html