S3-compatible Storages

Supported providers

For Dovecot Pro edition, only AWS S3 is officially supported. Please see Amazon S3.

This document covers the S3 compatible storage services, which are not supported.

plugin {
  # Basic configuration:
  obox_fs = s3:https://ACCESSKEY:SECRET@s3.example.com/?parameters

The parameters are:



See HTTP-Based Object Storages for common parameters


Specify region name for AWS S3 bucket. When this is specified, Dovecot starts using v4 signatures. (The default is to use v2 signatures.)


Perform AWS IAM lookup using this role name. See Amazon S3 for details.


IAM hostname and port. The default is Normally there is no reason to change this. This is mainly intended for testing.

Bucket name

There are two ways to specify the bucket name:

  • path-style: S3 requests’ path begins with the bucket parameter. This is configured with the bucket parameter. For example: https://s3.example.com/?bucket=BUCKETNAME will result in requests looking like https://s3.example.com/BUCKETNAME/object-path

  • virtual-hosted-style: The first subdomain in the URL specifies the bucket. AWS supports only this style for new buckets. For example For example https://BUCKETNAME.s3.example.com

S3 Example Configuration

Below is the configuration for a generic S3 storage. See Amazon S3 for AWS S3 specific example configurations.

mail_location = obox:%8Mu/%u:INDEX=~/:CONTROL=~/
plugin {
  obox_fs = fscache:512M:/var/cache/mails/%4Nu:compress:zstd:3:s3:https://ACCESSKEY:SECRET@s3.example.com/?bucket=mails
  obox_index_fs = compress:zstd:3:s3:https://ACCESSKEY:SECRET@s3.example.com/?bucket=mails
  fts_dovecot_fs = fts-cache:fscache:512M:/var/cache/fts/%4Nu:compress:zstd:3:s3:https://s3.example.com/%8Mu/%u/fts/?bucket=mails
  obox_max_parallel_deletes = 1000

We’ll use the first 8 characters of the hex representation of the MD5 hash of the username at the beginning of each object path. This is called dispersion prefix and is used by (at least) AWS S3 and GCS S3 to internally perform sharding and allow disk IO to scale.

S3 storage requirements

It’s important that the S3 storage has an efficient way to list objects with a given prefix. Copying performance is also important. Many S3 storages either don’t implement the listing at all, or it’s only used for disaster recovery type of purposes to list all objects. If this is the case, you can still use the storage together with fs-dictmap Configuration. See especially the storage-objectid-prefix and storage-passthrough-paths parameters.

Google Cloud Storage

GCS is similar to AWS in that a “dispersion prefix” is required to properly shard among the Google Cloud storage nodes. Google has provided verification that 6 characters of dispersion prefix is “enough distribution to ensure access to pretty massive resources on our side without gymnastics on our end.”

Deleting multiple objects per request

New in version 2.3.10.

The S3 drivers support bulk-delete requests. The bulk-delete option is enabled by default to delete up to 1000 keys with one request. To change this behaviour refer to bulk_delete_limit at HTTP-Based Object Storages. To actually delete that many mails in a single request, you must also set obox_max_parallel_deletes:

obox_max_parallel_deletes = 1000

This value should be the same as bulk_delete_limit or lower.