Delete messages older than a specific date in Exchange 2013

Hello.

My aim is to archive old emails to a pst file directly from Exchange 2013 cu3 server, without the need to do it manually on each Outlook client. I can export the messages, but I cannot successfully delete them. I tried with:

Search-Mailbox mailboxname-SearchQuery 'Received:<=12/31/2013 and kind:email' -DeleteContent
but it doesn't delete all the emails. With the following cmdlet I can successfully list all the messages that should be deleted:
New-MailboxSearch -Name estimate -SourceMailboxes mailboxname -SearchQuery 'received<"2012-12-31" AND kind:email' -EstimateOnly

Unfortuntaly New-MailboxSeach doesn't have a -DeleteContent switch, as far as I know, and if I use the same "SearchQuery" string in Search-Mailbox, the result isn't correct. Does anyone have a clue about it please? I'm really frustrated...

Thank you!