When a list message is sent to and received from the A-MQ6.2.1 broker, the received message contains List with one element that is the originally sent List (i.e:: if I send ['String'] in the message content the received massage has [['String']] in its content). Content_type used is 'amqp/list'.

This is some kind of regression as A-MQ6.3.0 and Qpidd brokers do not suffer from that.

There is a Formatter class that do the output for the QE clients and the List object is obtained by msg.getContentObject().asList() API call. If this call is done on message in the sender code, the list is retrieved as expected ['String'], but when the call is done in receiver code it outputs [['String']].

I'm not completely sure where the issues lies. When testing with A-MQ6.3.0 and Qpidd brokers the List content is received as expected, that implies it shall be A-MQ6.2.1 broker issue. On the other hand when the message (sent by qpid-cpp client to A-MQ6.2.1) is received with other client (qpid-jms, python-proton) the message is received as expected, that point to client issue. If it's a client library issue please feel free to change component accordingly.