The received lines on the server will show PST / PDT (with offsets from GMT). However, the date displayed in the “date” header is set by the client.
You don’t mention what mail client you’re using, and that would be the deciding factor. Assuming you’re using webmail, you just need to configure the time zone properly. You can do this in Options -> Personal Information.
If you’re using Pine or another console based mailer, you just need to set the TZ environment variable correctly.