You should be able to use Outlook which is included in WM6 (and WM5) without needing to purchase other software.
I have done this with gmail - although I prefer the online version on my phone (T-Mobile Vario 3 = HTC TyTnII). The address for yahoo mobile is m.yahoo.com
All the POP/SMTP settings are within options but are:
Incoming Mail Server (POP3): pop.mail.yahoo.co.uk
Use SSL, port: 995
Outgoing Mail Server (SMTP): smtp.mail.yahoo.co.uk
Use SSL, port: 465, use authentication
Account Name/Username: *******
Email address: [email protected]
Password: Your Yahoo! Mail password
Enter these into Outlook on your WM6 device and it should work, either when docked or via 3G/GPRS - be wary unless you have a reasonable data tarif.
Is this useful or do you need more than Outlook has to offer?