Some email services and Internet Service Providers use multiple mail servers for a single DNS name in a round-robin fashion such that PGP Desktop may create multiple messaging services for a single email account, seeing each mail server as separate and thus requiring its own messaging service.
PGP Desktop ships with wildcard support for common email services, such as *.yahoo.com and *.me.com (or *.mac.com). However, if you are using a less-common email service or if the services change their mail server configurations, you could run into this problem.
If you see PGP Desktop create multiple services for a single email account, and you check the settings and see they are the same except the mail server for the first service is mail1.example.com, the mail server for the second service is mail2.example.com, and the mail server for the third is mail3.example.com, and so on, you may need to manually edit one of the services.
The best solution is to manually edit one of the services such that the mail server entry for that service can support multiple mail servers being used round-robin. For the example cited above, you could manually change the server name on the Server Settings dialog box for one of the services to mail*.example.com, and then delete the other services.
Some round-robin setups may be more complicated, requiring a slightly different solution. For example, if PGP Desktop were to create services with mail servers of pop.frodo.example.com, smtp.bilbo.example.com, and mail.example.com, then the best wildcard solution would be *.example.com.