that sounds like s3 API based backend. also note that dovecot comes with some "non local mail storage plugins" in the commercial part:
```
In this release, we support Windows Azure, Amazon S3, Scality and
Dropbox systems.
```
so that should come close to what you want. for a local version you
could use e.g. radosdb/ceph or riak. both can provide the s3 API for
you.
hth
darix
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