In a previous post, I talked about troubleshooting hard bounces. In this one, I'm going to talk a bit about soft bounces, specifically soft bounces due to throttling.
What is throttling?
Throttling is an issue that occurs primarily with the large ISPs such as AOL, Yahoo, and Hotmail. When the ISPs are experiencing high volumes, they will throttle bulk email senders such as Marketo. It works kind of like this:
Marketo walks up to ISP with a big basket of emails.
Marketo: Hi, I'm here to deliver these emails--
ISP: I'm busy. Come back later.
(Two minutes later)
Marketo: Hi, I'm back to deliver these emails?
ISP: I'm busy...Fine! I'll take this many. (Grabs a handful of email.)
Marketo: What about the rest?
ISP: Come back later.
(Four minutes later)
Marketo: Hi, I'm back to deliver these emails?
IP: I'll take this many. (Grabs a handful of email.) Come back later.
(Eight minutes later)
...The cycle repeats with the time interval doubling each time.
Marketo is pretty patient, it will keep coming back and trying to deliver for 24 hours (36 hours for AOL). While it is still attempting to get the ISP to accept delivery on the emails, the emails will show in reporting as status "Pending." Once the 36 hours are up, Marketo will give up and bounce or soft bounce the emails as appropriate.
The challenge with throttling is that it's like weather or traffic congestion - there's not a lot you can do about it. One strategy is to break out your sends into smaller blocks - if you think of Marketo with the basket of email, it might be better to have multiple baskets and hope that the ISP takes a handful from each one. If you have a large number of leads on the commercial ISPs, you might even want to break each one out to a separate send. This doesn't necessarily help with the throttling, but it does give you better visibility to possible throttling issues that may be occurring on a specific ISP.
A third strategy is to try and massage your sender reputation, to make the ISP more likely to accept your email. To go back to my nightclub analogy, if you have a bunch of girls who want to get into the nightclub together, they usually send the cutest one of the group to go charm the doorman. From an email perspective, you want to send to your most engaged leads first - people who are likely to open and click on your emails, improving your reputation. The next day, you send your somewhat engaged leads, and the day after that, your least engaged leads.
If you're seeing persistent throttling, evidenced by high rates of pending mail and subsequent 'soft bounces' once the retry window is over, please reach out to support. Our deliverability team may be able to mediate on your behalf with the mailbox provider.
If you are in the B2B space and don't deal much with the large consumer ISPs, the chances are that throttling won't have a big impact on your deliverability. In the B2C space, however, throttling can definitely step on your overall email performance. Setting up reporting to keep an eye on your emails and trying different strategies to mitigate the effect can be an important part of your email marketing practice.
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