With the default policies left in effect, PGP will send email unencrypted to anyone who does not have a PGP key to encrypt to, just as if you had not installed PGP.
If you are using multiple email addresses, PGP will automatically set up and use a PGP Messaging Service for each of these. However, you can either disable this by disabling Discover New Accounts on the Messaging tab of PGP Options, after using the email address you want used. Or you can let PGP create messaging services for all email accounts, and then right click on each you don't want used, and select Diable - this will result in PGP ignoring email to and from those email addresses, and processing it just as if PGP was not installed.