Setting up no-delete one-way synchronization

[dej]

Hi guys,

so I just found out that I fundamentally misunderstood how offlineimap works and 
I need your help in finding out wether I can make this work with offlineimap or 
if I need to use some other tools.

I have a mail account with Riseup.net. To save space, users only get to keep 
the emails of the past three months in the Sent folder.

Now I thought I could deal with that by having a local copy of the whole IMAP 
folder structure, however, offlineimap deletes everything locally that is 
deleted remotely.

I would like to do a simple backup, I don't ever do anything with the local 
Maildir folder. I just want it to be there so I can look up mail that is older 
than three months.

Any chance?

I already know about the realdelete parameter but that does not seem to do 
what I want.

Cheers,

dej