The goal of Sparrow Mailing is to deliver your campaigns as quickly as possible. Technically, our system can deliver a million emails in 45 minutes, but large ISPs group and defer deliveries as a spam-prevention measure. A number of factors determine which campaigns ISPs throttle and for how long.

There is no authoritative public source on the limits ISPs use, but several factors need to be taken into account.

• high bounce rates
• spam complaints
• subscriber engagement, including open rates, click rates and time spent reading emails
• spam trap hits

The limits applied by the main ISPs can change if suspicious behaviour is detected. When an ISP notices five bounces within a certain time, it sends a throttling message telling the sending server to slow down or stop sending emails for a while. If this message is ignored, the ISP may simply decide to block emails sent from that IP address.

For new IP addresses, ISPs generally limit the number of emails sent from that address until they can determine the type, volume and general characteristics of the emails sent.

The key factors are consistency and quality. A new IP address will therefore need to send emails regularly for a while before the ISP allows more emails per hour or per day.

If you send emails to one of the main ISPs that throttles email, Sparrow Mailing will send your campaign from several IP addresses to account for the throttling. There is no box to tick or setting to change; everything happens automatically on our servers.

With this in mind, it is not impossible to introduce artificial delays into the sending process, or into a specific user's campaigns. Likewise, for business or personal domains, it is impossible to say how fast we send to those domains.