I am not sure if I understand your remark. Retain does not need any agents on PO servers. Retain will get its data via SOAP.

The documentation I have been reading says the best practice is to place the worker agents on the Post Office servers. Second, is build a separate server just for worker agents.  Third, if you have less than 500 mailboxes, you can use the Retain server server. The docs do mention using Reload to pull the messages.  I do not want to over complicate this installation, but because I have never used Retain before I really don't know if It can handle running all services from one server for 2800 mailboxes.

Well, you're right. I did not ask for your environment size.

However two of my Retain environments are universities. So they have to support more than 4000 mailboxes I think. But more important is how many "heavy" mailboxes do you have to support. I.e. do you have a lot of students which are "light" mailboxes? Usually students do not user GroupWise clients which causes smaller accounts and less dynamics. 

Coming back to my example. Both universities are using one Retain server and no local agents. University A has three post offices, so we use only one "main" worker (and one for additional purposes or to read local GW archives). University B has more than 10 post offices; here we have decided to run 4 workers to support concurrent activities. University A runs a job once a week (9 - 10 hours), university B runs daily archive jobs which need only a few minutes.

Both universities are using Reload too. But  Retain will work with POAs directly.

Just to clarify "Both universities are using one Retain server and no local agents" No agents on the PO servers or No agents on the Retain server?

Thanks again

Do you mean worker threads? In both cases Retain did run some (2 up to 4 or 5) worker threads.

Got it.  Thanks Diethmar.