Hi Michal,

The following technique are used for incoming mail:

>> if replies message to an existing thread by having a Message-Id that matches an existing mail_message.message_id, we take the original message model/thread_id pair and ignore custom_value as no creation will take place.

>> if the message replies to an existing thread by having In-Reply-To or References matching odoo model/thread_id Message-Id and if this thread has messages without message_id, take this model/thread_id pair and ignore custom_value as no creation will take place.

>> look for a mail.alias entry matching the message recipients and use the corresponding model, thread_id, custom_values and user_id. This could lead to a thread update or creation depending on the alias.

>> fallback on provided "model", "thread_id" & "custom_values".

>> raise an exception as no route has been found.

Also see is this helps

Hi Michal,

For incoming mail, if the contact receives the message and then replies but gets the error message "Delivery Incomplete - There was a temporary problem when delivering your message to... Gmail will retry for 47 more hours." then your server is misconfigured.  You need to ensure that email sent from Odoo have a valid "reply-to" set and that your server can receive email at that address.