Hard vs. Soft Bounces
Traditionally, a “hard bounce” indicates a permanent reason an email cannot be delivered. The usual reasons are an invalid email or an invalid domain.
“Soft bounces” have been defined as temporary failures due to an unavailable server, a full mailbox, or an oversize message. While those causes still hold true in theory, they no longer represent the top reasons behind today’s soft bounces, which are mostly caused by ISP blocks. In most cases the blocks are temporary and can often be resolved by identifying the root causes, then following email best practices and ISP guidelines.
Here are six of those reasons, with suggested remedies:
1. A High Hard Bounce Rate Can Trigger Soft Bounces
Having too many invalid recipients – i.e., a high hard bounce rate – is a clear sign to ISPs that you are sending to a poor mailing list and may consequently incur a temporary block. At Act-On we consider a high hard bounce rate to be anything higher than 1%. The most effective way to reduce the number of hard bounces is by cleaning your list regularly of invalid addresses through an email validation service.
2. Too Many Messages Sent
Volume consistency is a vital component in deliverability, and ISPs are always on the alert for any irregular sending activity such as spikes in email volume. AOL, for example, tends to block or delay non-whitelisted IP addresses when they detect such unfavorable statistics. Fortunately, if you have a good reputation with AOL, you can apply to be whitelisted. If not, then it’s time to segment your list and maintain a consistent sending schedule. First segment your list by engaged and unengaged recipients. Then stagger your sends to the unengaged over a time length you specify, for example, four hours. (Act-On can manage this staggered send for you.)
3. Spam Traps Hit
This is another way for ISPs to determine the reputation of a sender – and another reason why list hygiene is so important. Most spam traps can be identified and removed by a data cleaning service. If not, they are usually hidden in your inactive list, as they generally don’t engage with any email. By suppressing the unengaged segment, one of my clients recently was able to resolve a temporary block at Comcast that used to occur every single time they sent to their Comcast recipients.
4. Too Many Spam Complaints
Spam complaints are probably the most negative engagement signal you can ever generate from your email. Some ISPs, such as Yahoo, will clearly state in their bounce message that your email is temporarily deferred due to user complaints.