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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 address is on any blacklists. You could use or any other similar service to check for your IP address.
  • 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! Docker and iptables-based firewalls sometimes create conflicting rules, so disable the firewall on your host to determine whether your connection issues are caused by such conflicts. If they are, you need to manually create appropriate rules in your host firewall to permit the necessary connections.

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 PowerShell command Test-NetConnection -ComputerName host -Port port available by default.

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

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.

Dovecot keeps restarting (see #2672)

Check that you have at least the following files in data/assets/ssl:


If dhparams.pem is missing, you can generate it with

openssl dhparam -out data/assets/ssl/dhparams.pem 4096

Rspamd reports: cannot open hyperscan cache file /var/lib/rspamd/{...}.hs: compiled for a different platform

When migrating mailcow to another system (usually with a different CPU), Rspamd may report that it cannot load some (possibly all) .hs files because they were compiled for a different platform (CPU).

This is related to Hyperscan2, an Intel technique for precompiling regex schemas that Rspamd uses.

This feature provides a significant performance boost and is therefore heavily embedded in Rspamd.

To fix this bug, all .hs and .hsmp files must be deleted from the rspamd directory:

cd MAILCOW_ROOT # Usually /opt/mailcow-dockerized
docker compose exec rspamd-mailcow bash
rm -rf /var/lib/rspamd/*.hs
rm -rf /var/lib/rspamd/*.hsmp
cd MAILCOW_ROOT # Usually /opt/mailcow-dockerized
docker-compose exec rspamd-mailcow bash
rm -rf /var/lib/rspamd/*.hs
rm -rf /var/lib/rspamd/*.hsmp

Restart Rspamd afterwards:

docker compose restart rspamd-mailcow
docker-compose restart rspamd-mailcow

Now Rspamd recompiles the said regex maps again with Hyperscan.