Messages sent to custom domains and private addresses, held by customers on a paid plan, are throttled differently than messages sent to public addresses in the mailsac.com domain.
Throttling of messages on custom domains and private addresses will likely be completely unnoticed by customers, with the exception of customers who are sending messages at the rate of thousands per minute.
All emails received by Mailsac are subject to throttling to ensure system availability for all customers, both paying and free.
Throttling on Public / Unowned Domains
Messages sent to public / unowned domains have a lower threshold for throttling. This includes public @mailsac.com addresses and domains that have been configured to deliver mail to Mailsac, but are not verified.
To increase the throttling threshold a Private Address or Verified Custom Domain should be be used.
Throttling on Private Addresses or Custom Domains
Most customers sending to Private Addresses or a Verified Custom Domain will never experience throttling.
How do I know if I’m Being Throttled?
There are two stages to throttling.
1. Message deliver is slowed
During this stage messages are processed increasingly slower based on the number of emails received. This stage is done without customer notification.
In a worse case scenario, this will result in messages being delayed about 1 minute.
2. Mailsac SMTP server returns a 421 error message
A 421 error message indicates the number of messages received has exceeding the throttling limits and message delivery will need to be retried at a later time.