What are your sending limits?
Email accounts are automatically suspended for sending more than 300 emails in the current hour. That does not mean that it is okay to send 300 emails per hour. You should be sending quality emails, necessary emails, and emails that people want to receive.
More specific limit values:
100 per recipient, per hour
300 per sender, per hour
Limits subject to change as needed to address trends that cause problems for our customers.