Hello everybody,
I was in same situation, all email notification were in queue, I found a issue with my email user. In my case the cron use the OS user account www-data and it tried to connect to email service with email account www-data@domain.com, account not configured. I just add an alias in my email service www-data@domain.com --> exist_account_user@domain.com and it was my solution. I hope this be useful.
