Seems like you’re leaving information out here.
SENDER_QUOTA_REJECTION is supposed to mean you went over the limit of recipients per hour, which via sendmail interface is 200. SMTP is 100 an hour. This quota enforced hourly so it should never be “stuck” for more than an hour or two at a time given you’ve stopped trying to send messages. Now the confusing thing is you would get this same message if you’re not using a valid From header.
So that error message itself is not a bug. There may very well be a bug that triggers that error message when otherwise it shouldn’t, but nothing you’ve said explains your situation either way. Have you determined how many messages you send per hour? What else did Support say? Have you tried setting up Batch sending, with something like 15 messages every 6 minutes?
What exactly are you confused about?
Authentication: SMTP Authentication (SASL)
Port: 587 or 25
Customer since 2000 openvein.org