Before you run **mailcow: dockerized**, there are a few requirements that you should check:

!!! warning
    When running mailcow: dockerized on a Debian 8 (Jessie) box, you should [switch to kernel 4.9 from Jessie backports](https://packages.debian.org/jessie-backports/linux-image-amd64) to avoid a bug when running Docker containers with *healthchecks*! For more details read: [github.com/docker/docker/issues/30402](https://github.com/docker/docker/issues/30402)

!!! info
    - mailcow: dockerized requires [some ports](#default-ports) to be open for incoming connections, so make sure that your firewall is not blocking these.
    - Make sure that no other application is interfering with mailcow's configuration, such as another mail service
    - A correct DNS setup is crucial to every good mailserver setup, so please make sure you got at least the [basics](../prerequisite-dns#the-minimal-dns-configuration) covered before you begin!
    - Make sure that your system has a correct date and [time setup](#date-and-time). This is crucial for stuff like two factor TOTP authentication.

## Minimum System Resources

Please make sure that your system has at least the following resources:

| Resource                | mailcow: dockerized                          |
| ----------------------- | -------------------------------------------- |
| CPU                     | 1 GHz                                        |
| RAM                     | 2 GiB + Swap (better: 4 GiB and more + Swap) |
| Disk                    | 15 GiB (without emails)                      |
| System Type             | x86_64                                       |

ClamAV and Solr are greedy RAM munchers. You can disable them in `mailcow.conf` by settings SKIP_CLAMD=y and SKIP_SOLR=y.

## Firewall & Ports

Please check if any of mailcow's standard ports are open and not in use by other applications:

```
# netstat -tulpn | grep -E -w '25|80|110|143|443|465|587|993|995'
```

!!! warning
    There are several problems with running mailcow on a firewalld/ufw enabled system. You should disable it (if possible) and move your ruleset to the DOCKER-USER chain, which is not cleared by a Docker service restart, instead. See [this blog post](https://blog.donnex.net/docker-and-iptables-filtering/) for information about how to use iptables-persistent with the DOCKER-USER chain.
    As mailcow runs dockerized, INPUT rules have no effect on restricting access to mailcow. Use the FORWARD chain instead.

**

If this command returns any results please remove or stop the application running on that port. You may also adjust mailcows ports via the `mailcow.conf` configuration file.

### Default Ports

If you have a firewall in front of mailcow, please make sure that these ports are open for incoming connections:

| Service             | Protocol | Port   | Container       | Variable                         |
| --------------------|:--------:|:-------|:----------------|----------------------------------|
| Postfix SMTP        | TCP      | 25     | postfix-mailcow | `${SMTP_PORT}`                   |
| Postfix SMTPS       | TCP      | 465    | postfix-mailcow | `${SMTPS_PORT}`                  |
| Postfix Submission  | TCP      | 587    | postfix-mailcow | `${SUBMISSION_PORT}`             |
| Dovecot IMAP        | TCP      | 143    | dovecot-mailcow | `${IMAP_PORT}`                   |
| Dovecot IMAPS       | TCP      | 993    | dovecot-mailcow | `${IMAPS_PORT}`                  |
| Dovecot POP3        | TCP      | 110    | dovecot-mailcow | `${POP_PORT}`                    |
| Dovecot POP3S       | TCP      | 995    | dovecot-mailcow | `${POPS_PORT}`                   |
| Dovecot ManageSieve | TCP      | 4190   | dovecot-mailcow | `${SIEVE_PORT}`                  |
| HTTP(S)             | TCP      | 80/443 | nginx-mailcow   | `${HTTP_PORT}` / `${HTTPS_PORT}` |

To bind a service to an IP address, you can prepend the IP like this: `SMTP_PORT=1.2.3.4:25`

**Important**: You cannot use IP:PORT bindings in HTTP_PORT and HTTPS_PORT. Please use `HTTP_PORT=1234` and `HTTP_BIND=1.2.3.4` instead.

## Date and Time

To ensure that you have the correct date and time setup on your system, please check the output of `timedatectl status`:

```
$ timedatectl status
      Local time: Sat 2017-05-06 02:12:33 CEST
  Universal time: Sat 2017-05-06 00:12:33 UTC
        RTC time: Sat 2017-05-06 00:12:32
       Time zone: Europe/Berlin (CEST, +0200)
     NTP enabled: yes
NTP synchronized: yes
 RTC in local TZ: no
      DST active: yes
 Last DST change: DST began at
                  Sun 2017-03-26 01:59:59 CET
                  Sun 2017-03-26 03:00:00 CEST
 Next DST change: DST ends (the clock jumps one hour backwards) at
                  Sun 2017-10-29 02:59:59 CEST
                  Sun 2017-10-29 02:00:00 CET
```

The lines `NTP enabled: yes` and `NTP synchronized: yes` indicate wether you have NTP enabled and if it's synchronized.

To enable NTP you need to run the command `timedatectl set-ntp true`. You also need to edit your `/etc/systemd/timesyncd.conf`:

```
# vim /etc/systemd/timesyncd.conf
[Time]
Servers=0.pool.ntp.org 1.pool.ntp.org 2.pool.ntp.org 3.pool.ntp.org
```

## Hetzner Cloud (and probably others)

Check `/etc/network/interfaces.d/50-cloud-init.cfg` and change the IPv6 interface from eth0:0 to eth0:

```
# Wrong:
auto eth0:0
iface eth0:0 inet6 static
# Right:
auto eth0
iface eth0 inet6 static
```

Reboot or restart the interface.
You may want to [disable cloud-init network changes.](https://wiki.hetzner.de/index.php/Cloud_IP_static/en#disable_cloud-init_network_changes)

## MTU

Especially relevant for OpenStack users: Check your MTU and set it accordingly in docker-compose.yml. See **4.1** in [our installation docs](https://mailcow.github.io/mailcow-dockerized-docs/install/).