An email program that does not decrypt and sign mail (using S/MIME and OpenPGP); does not send alerts to Notification Center; and does not allow you to search your emails' full (including formatted) text: that is not an email program you would consider?
Now, you can consider MailMate. Version 1.5 also brings colored flags (that synchronize with OS X Mail via most IMAP accounts) and can complete recipients using LDAP.
›› MailMate is a refreshingly efficient way to deal with mail in IMAP accounts, all with handy keyboard shortcuts, superior search and precise smart folders. (Mac)