How to use mdbox format instead of maildir format

mejay · Friday at 3:44 PM

Hi,

What is the best/proper way to setup dovecot to use mdbox format instead of maildir format in DirectAdmin?

Thank you.

zEitEr · Monday at 1:02 AM

Hello,

Mbox format is deprecated and should not be used in production setups at all.

More details: https://doc.dovecot.org/main/core/config/mailbox_formats/mbox.html

And I am not even sure, the setup is possible under Dovecot+Exim+DirectAdmin

mejay · Monday at 6:52 AM

Yes, mbox is no longer used, but I'm asking about mdbox which is completely different and is widely used and supported.

SEO Guru · Monday at 1:03 PM

Mdbox is prone to corruption. Also modern backup solutions dont duplicate files so the backup footprint gets smaller.

Stick to Maildir!

zEitEr · 2025-12-23T04:25:09-0700

mejay said:
Yes, mbox is no longer used, but I'm asking about mdbox which is completely different and is widely used and supported.

You might check the official docs for this:

- https://doc.dovecot.org/2.4.2/core/config/mailbox_formats/dbox.html
- https://doc.dovecot.org/2.4.2/core/config/mailbox_formats/dbox.html#configuration

I don't have instructions on how to convert existing setup from maildir to mdbox. Probably other guys have anything. If they don't, you will need to figure it out on your own. Or hire somebody (I am available as well) for this conversion.

See these pages for an information on how to customize Exim/Dovecot on a directadmin server:

- https://docs.directadmin.com/getting-started/first-steps/faq.html#how-to-customize-exim-dovecot
- https://docs.directadmin.com/other-.../customizing-dovecot.html#customizing-dovecot

The pages might miss step-by-step instructions, but one can figure everything out by careful reading.

p.s. happy holiday season and new year

johannes · 2025-12-23T14:37:15-0700

plz dont beat me to post here some AI answers on this topic ..

Difference Between mbox and mdbox

Here’s a clean, technical comparison of the two mailbox storage formats, especially as used in Dovecot.

Core Idea

mbox = one single file per mailbox folder, all emails stored sequentially inside it.
mdbox = Dovecot’s modern format, storing multiple messages per file, distributed across several files for each mailbox.

Comparison Table

Format	Structure	Performance	Reliability	Storage Efficiency	Best Use Case
mbox	One large file per folder	Slow with large mailboxes	Vulnerable to corruption	Inefficient with many small emails	Legacy UNIX systems
mdbox	Multiple messages per file	Very fast (index‑optimized)	Robust due to Dovecot indexing	Highly efficient	Modern Dovecot installations

Technical Details

mbox

Stores all messages of a folder in one big file.
Any change (delete, move, append) requires rewriting parts of the file.
Performance degrades heavily as the mailbox grows.
If the file becomes corrupted, the entire mailbox folder is at risk.
Very old, traditional UNIX format.

mdbox

Part of Dovecot’s dbox storage family.
Stores multiple messages per file, but splits them across several files.
Uses Dovecot’s index files for extremely fast access.
Much more efficient for:
- large mailboxes
- IMAP usage
- backups
- servers with many users
More resilient to corruption — only small chunks are affected, not the whole mailbox.

When to Use What

Use mbox only if you absolutely must

Legacy systems
Compatibility with old tools
Not recommended for modern mail servers

Use mdbox for modern setups

Dovecot servers
High‑performance IMAP
Large or growing mailboxes
Efficient storage and fast indexing

Summary

mdbox is superior to mbox in almost every way: faster, safer, more efficient, and designed for modern mail servers.
mbox is essentially a legacy format that persists only for compatibility reasons.

Here is a clear, safe, step‑by‑step explanation of how to migrate from mbox → mdbox in Dovecot, written so you can follow it without surprises.
This is the officially recommended migration path used by most modern mail server admins.

Below is the standard, safe, supported method using Dovecot’s built‑in migration tools.

1) Before migrating, you need to add mdbox as a new storage location, but keep mbox active so Dovecot can read from it.

Edit your Dovecot config (usually

mail_location = mbox:~/mail:INBOX=/var/mail/%u mail_home = /home/%u 

Change it to:

mail_location = mdbox:~/mdbox mail_home = /home/%u namespace inbox { inbox = yes } 

But do not remove the old mbox settings yet — you will still need them for the migration command.

2) For each user:

mkdir /home/USERNAME/mdbox chown USERNAME:USERNAME /home/USERNAME/mdbox chmod 700 /home/USERNAME/mdbox

3) Dovecot includes a built‑in command to copy mail from mbox → mdbox:

doveadm -Dv backup -u USERNAME mdbox:~/mdbox 
What this does:

Reads the user’s existing mbox mail
Writes it into mdbox format
Preserves flags, folders, and indexes

You can run this for all users:
for u in $(doveadm user '*'); do doveadm -Dv backup -u "$u" mdbox:~/mdbox done

4) Once migration is complete and verified, update your config:

mail_location = mdbox:~/mdbox 
Remove or comment out the old mbox settings.

Then reload Dovecot:
systemctl reload dovecot

5)

Use:
doveadm mailbox list -u USERNAME doveadm fetch -u USERNAME 'hdr.subject' mailbox INBOX 

Check:

All folders exist
All messages are present
Flags (Seen, Answered, etc.) are correct

6) After you confirm everything works:

rm -rf ~/mail rm /var/mail/USERNAME 
But only after backups.

7)

mdbox benefits from Dovecot’s index optimizations:

mail_prefetch_count = 20 mdbox_rotate_size = 2M mdbox_rotate_interval = 1d 

This keeps mdbox files small and corruption‑resistant.

Migrating from mbox → mdbox involves:

Preparing Dovecot config
Creating mdbox directories
Running doveadm backup
Switching mail_location
Verifying
Cleaning up old mbox files

mdbox gives you:

Faster IMAP
Smaller storage footprint
Better reliability
Faster backups
Less I/O load

Richard G · 2025-12-23T14:45:59-0700

johannes said:
post here some AI answers

Well... won't beat you but would strongly advise against it.
Reason is that anybody can find an AI answer easy enough themselfs, that is not where forums are for. Just as well you could post Google answers.

Most important reason, still often AI gives a wrong answer or partly wrong answer, especially with commands and settings.
So it still can't be trusted 100% without additional confirmation. And if confirmation is needed, then one can best get an answer from good sites than an AI.

johannes · 2025-12-23T15:22:22-0700

Is the conversion path wrong? Did you tried it? Its up to the OP to test it. At least it gives a direction and hints, to enjoy with a grain of salt..

Richard G · 2025-12-23T16:17:26-0700

johannes said:
Is the conversion path wrong? Did you tried it?

Did you?
I didn't say that did I? I just warned about often AI gives wrong answers. So people have to take care and think about that.

zEitEr · 2025-12-24T00:54:50-0700

johannes said:
Is the conversion path wrong? Did you tried it? Its up to the OP to test it. At least it gives a direction and hints, to enjoy with a grain of salt..

Johannes,

Thank you for the input. First of all I would rather follow official docs and instructions from the official site. They have more trustworthy.

Then the suggested instructions:

- are not compatible with a setup from Directadmin (wrong paths and locations)
- seems to be written for Dovecot 2.3.x., and Dovecot 2.4.x has different config variables and syntax
- won't work with virtual users

Dovecot 2.4.x does not have mail_location directive any longer. It does not exist on the official page https://doc.dovecot.org/2.4.2/core/summaries/settings.html#all-dovecot-settings

and does not exist in the config for Dovecot 2.4.x:

Bash:

[root@poralix1 ~]# dovecot --version
2.4.2 (0962ed2104)
[root@poralix1 ~]# doveconf -n| grep mail_location
[root@poralix1 ~]#

but can be found in the config for Dovecot 2.3.x:

Bash:

[root@poralix2 ~]# dovecot --version
2.3.21 (47349e2482)
[root@poralix2 ~]# doveconf -n| grep mail_location
mail_location = maildir:~/Maildir
[root@poralix2 ~]#

This is a quick overview. Correct me if I'm wrong, but AI solutions might bring to data loss and are too risky to follow.

How to use mdbox format instead of maildir format

mejay

Verified User

zEitEr

Super Moderator

mejay

Verified User

SEO Guru

Verified User

zEitEr

Super Moderator

johannes

Verified User

Difference Between mbox and mdbox

Core Idea

Comparison Table

Technical Details

mbox

mdbox

When to Use What

Use mbox only if you absolutely must

Use mdbox for modern setups

Summary

Below is the standard, safe, supported method using Dovecot’s built‑in migration tools.

1) Before migrating, you need to add mdbox as a new storage location, but keep mbox active so Dovecot can read from it.

Edit your Dovecot config (usually

2) For each user:

3) Dovecot includes a built‑in command to copy mail from mbox → mdbox:

4) Once migration is complete and verified, update your config:

5)

6) After you confirm everything works:

7)

Richard G

Verified User

johannes

Verified User

Richard G

Verified User

zEitEr

Super Moderator

How to use mdbox format instead of maildir format

Verified User

Super Moderator

Verified User

Verified User

Super Moderator

Verified User

Difference Between mbox and mdbox​

Core Idea​

Comparison Table​

Technical Details​

mbox​

mdbox​

When to Use What​

Use mbox only if you absolutely must​

Use mdbox for modern setups​

Summary​

Below is the standard, safe, supported method using Dovecot’s built‑in migration tools.​

1) Before migrating, you need to add mdbox as a new storage location, but keep mbox active so Dovecot can read from it.​

Edit your Dovecot config (usually​

2) For each user:​

3) Dovecot includes a built‑in command to copy mail from mbox → mdbox:​

4) Once migration is complete and verified, update your config:​

5)​

6) After you confirm everything works:​

7)​

​

Verified User

Verified User

Verified User

Super Moderator

Difference Between mbox and mdbox

Core Idea

Comparison Table

Technical Details

mbox

mdbox

When to Use What

Use mbox only if you absolutely must

Use mdbox for modern setups

Summary

Below is the standard, safe, supported method using Dovecot’s built‑in migration tools.

1) Before migrating, you need to add mdbox as a new storage location, but keep mbox active so Dovecot can read from it.

Edit your Dovecot config (usually

2) For each user:

3) Dovecot includes a built‑in command to copy mail from mbox → mdbox:

4) Once migration is complete and verified, update your config:

5)

6) After you confirm everything works:

7)