Common Problems

Here we list common problems and possible solutions:

Mail loops back to myself

Please check in your mailcow UI if you made the domain a backup MX:

Check your MX Backup settings

I can receive but not send mails

There are a lot of things that could prevent you from sending mail:

  • Check if your IP is on any blacklists. You could use dnsbl.info or any other similar service to check for your IP.
  • There are some consumer ISP routers out there, that block mail ports for non whitelisted domains. Please check if you can reach your server on the ports 465 or 587:
# telnet 465
Connected to
Escape character is '^]'.

My mails are identified as Spam

Please read our guide on DNS configuration.

docker-compose throws weird errors

... like:

  • ERROR: Invalid interpolation format ...
  • AttributeError: 'NoneType' object has no attribute 'keys'.
  • ERROR: In file './docker-compose.yml' service 'version' doesn't have any configuration options.

When you encounter one or similar messages while trying to run mailcow: dockerized please check if you have the latest version of Docker and docker-compose

Container XY is unhealthy

This error tries to tell you that one of the (health) conditions for a certain container are not met. Therefore it can't be started. This can have several reasons, the most common one is an updated git clone but old docker image or vice versa.

A wrong configured firewall could also cause such a failure. The containers need to be able to talk to each other over the network

It might also be wrongly linked file (i.e. SSL certificate) that prevents a crucial container (nginx) from starting, so always check your logs to get an Idea where your problem is coming from.

Address already in use

If you get an error message like:

ERROR: for postfix-mailcow  Cannot start service postfix-mailcow: driver failed programming external     connectivity on endpoint mailcowdockerized_postfix-mailcow_1: Error starting userland proxy: listen tcp bind: address already in use

while trying to start / install mailcow: dockerized, make sure you've followed our section on the prerequisites.

XYZ can't connect to ...

Please check your local firewall!

If you experience connection problems from home, please check your ISP router's firewall too, some of them block mail traffic on the SMTP (587) or SMTPS (465) ports. It could also be, that your ISP is blocking the ports for SUBMISSION (25).

While Linux users can chose from a variety of tools1 to check if a port is open, the Windows user has only the command telnet host port available by default (and it has to be activated since Windows Vista).

To enable telnet on a Windows after Vista please check this guide or enter the following command in an terminal with administrator priviliges:

dism /online /Enable-Feature /FeatureName:TelnetClient

Inotify instance limit for user 5000 (UID vmail) exceeded (see #453)

Docker containers use the Docker hosts inotify limits. Setting them on your Docker host will pass them to the container.