Plugins in Dovecot are really simple. They basically have two functions:

  • <plugin_name>_init(module) is called when module_dir_init() is called.

  • <plugin_name>_deinit() is called when module_dir_deinit() or module_dir_unload() is called.

The <plugin_name> is the short version of the plugin name, based on the filename. For example if the filename is, the <plugin_name> is imap_quota and the init function to be called is imap_quota_plugin_init().


Since different Dovecot versions can have different APIs, your plugin should usually also define <plugin_name>_version, like:

const char *imap_quota_plugin_version = DOVECOT_ABI_VERSION;

If the version string in plugin doesn’t match the version of the running binary, the plugin loading fails. The DOVECOT_ABI_VERSION is defined in Dovecot’s config.h, which you’re typically including.

It’s possible to check the Dovecot version number with a macro. This allows either supporting different Dovecot APIs or giving a clear error message if the API is too old to support your plugin. For example:

#if ! DOVECOT_PREREQ(2, 3, 18)
#  error Must have at least v2.3.18

Changed in version v2.3.18: added the 3rd micro-version parameter.


Some plugins depend on another one. In some systems (but not all) it’s possible to handle this by giving a nicer error message than “symbol xyz not found”. There are two steps for this:

First create <plugin_name>_dependencies array listing plugin names that the plugin depends on, like:

const char *imap_quota_plugin_dependencies[] = { "quota", NULL };

Then you’ll also have to make the plugin .so binary link to the other plugins:

lib11_imap_quota_plugin_la_LIBADD = \

PLUGIN_DEPS is set only if plugin dependencies are actually supported. Otherwise the build might fail or plugin loading might fail.

Once all this is done, trying to load imap_quota plugin without quota plugin gives a nice error message:

Error: Can't load plugin imap_quota_plugin: Plugin quota must be loaded also


Different kinds of plugins can also hook into various things:

  • imap: imap_client_created_hook_set()

  • pop3: pop3_client_created_hook_set()

  • submission: submission_client_created_hook_set()

  • lmtp: hook_client_created

  • lib-storage: mail_storage_hooks_add()