Odoo have it's own cron solution for triggering the tasks saved in the ir.cron model. Every cron definition define a call to a target model function that implements the needed behavior of the task. The target model function, in this case function for Email Queue Manager is the responsible for clean up the failed or delayed email messages. The cron of Odoo consist of independents Threads/Workers that scan and execute the cron task definitions 

The process_email_queue function used by the referenced cron look for mail.mail objects with state = outgoing. Those emails that are not delivered get an state = exception. For those you could put back to the outgoing state by the Retry button or you could define the context in the args for that cron task with a filter key-value containing a domain to match the exception state like:

<field eval="'(None,{\'filters\': [\'|\'(\'state\', \'=\', \'exception\')]})'" name="args"/>

or directly in the ir.cron form in the field args

"(None,{'filters': ['|'('state', '=', 'exception')]})"