Add this to the transports section of your Exim config file, i.e. somewhere between the first and second ``end'' line:
mailman_transport: driver = pipe command = MAILMAN_WRAP \ '${if def:local_part_suffix \ {${sg{$local_part_suffix}{-(\\w+)(\\+.*)?}{\$1}}} \ {post}}' \ $local_part current_directory = MAILMAN_HOME home_directory = MAILMAN_HOME user = MAILMAN_USER group = MAILMAN_GROUP