No more worry about the failed E-mails, install Auto Resend to resend the failed E-mails automatically.
Features:
By default, the whole Emails will be checked once a day
If there are any failed emails, then it will resend
To change the time period
Activate the developer mode, Then Settings -> Technical -> Automation -> Scheduled Action
Open the record named 'Auto Resend Emails'
Configure it here