I believe there is no automatic retry mechanism if an email is not sent due to communication limit reached.
Also there is no mechanism to set up priority due to that.
You might want to,
a> Increase your daily communication limit if this is frequent issue.
b> For select 'high priority' / 'urgent' email programs, modify 'program settings' to ignore communication limits for non-operational emails.
Couple of helpful urls
https://community.marketo.com/MarketoArticle?id=kA050000000LAoDCAW
Which explains that the daily limit is reseted at midnight at the timezone in which the Marketo instance is located.
Hope this helps
Rajesh