Email deliverability is the ability to deliver emails to subscribers’ inboxes, while email delivery refers to ability to deliver email to mailbox provider server. So when an email service providers (ESP) boast high deliverability they don’t usually mean high inbox rate. Emails delivered are emails not rejecting by the mailbox provider, so emails in a spam folder are considered delivered.

This article focuses on advanced deliverability topics, so make sure to review our email deliverability guide to best practices first.

What happens when an email isn’t accepted? A few things, like:

  1. Hard bounce means an email address is no longer valid. No future re-delivery attempts are made.
  2. Soft bounce can happen for several reasons:
    • Mailbox provider service is temporarily down.
    • Recipient mailbox is full.
    • Email is rejected. No further attempts to deliver are made automatically.
  3. Email delivery is delayed. Several attempts are made to re-deliver for up to two days, until the email is either rejected or delivered. The first attempt to re-deliver is made in about 20 min, then a few hours. This is why your campaign may appear either healthy or with low engagement, but the soft-bounce rate can jump the day or even two after a campaign is sent.

Common Rejection (Non-Delivery) Reasons:

  1. Higher than typical volume compared to recent history for a sending domain. Large list jumps of 50% or more from one day to the next indicate inorganic list growth due to list renting/purchasing. What’s a safe sending rate increase for a sender domain warmup? It depends on the starting point and how far along in the warmup your domain is.
    • For list sizes under 50K you could increase by 20-30%/day
    • For list sizes over 300K you should stay under 20% per day
    • A brand new domain should start at around 100-300 emails per day and can go up by 50% while under 10k/day, as long as the list is diverse – meaning it’s not 50%+ gmail/yahoo/aol only.
  2. Sender domain reputation – this is a combination of:
    • Hard bounce rate – historical and immediate.
    • Historical complain rate.
    • Sending to spam traps.
    • Reputation of the IP address sending emails. Unless you are paying for a private/dedicated IP address your emails go out from a shared IP pool, meaning a set of IPs used by many senders.
  3. Historical and recent low engagement. This can be split into 2 separate issues:
    • You may see 20-35% on your daily or weekly campaigns, but if it’s the same subscribers opening your emails and 75% of your list is completely inactive that’s a problem.  Mailbox providers don’t like seeing sending activity with no engagement, they see it as abusive and low quality practice.
    • You may see a 20-25% open rate for your campaigns, but upon closer inspection an engagement by mailbox providers report may show that your open rate is high for some providers and very low for others. If you start seeing an open rate drop below 3% for a specific mailbox provider, e.g. yahoo/aol, you need to make adjustments to your sending strategy to avoid rejections.

Tactics to Improve Deliverability and Inboxing

Remove Spam Traps

A common tactic to improve your sender reputation is to remove spam traps from your email lists. Spam traps are used to identify and monitor spam email by Internet Service Providers (ISPs), anti-spam organizations, and blocklist operators. They are email addresses (also called honeypots) used to identify senders who aren’t following email best practices. If you either purchase your email lists or scrape email addresses from the web there is a good chance you have a lot of spam traps on your list. Learn more about avoiding, identifying and removing spam traps in our article on spam traps. If you are a BigMailer customer you can reach out to us to help you determine if this is an issue for you and the most cost effective way to remove them.

Improve Engagement

The best way to address both low engagement and high complaint rate is to reduce sending frequency, especially to inactive subscribers on your list. When someone checks their email infrequently and they see 2+ daily emails from the same sender they are more likely to complain (report as Spam, or move to Spam folder) and they can flag every email from a sender, not just one.

Good send time is 6am-3pm local time Monday-Friday, while 7am-10am Tuesday-Thursday is typically best. That’s because most emails are checked during typical work hours. Avoid sending after 5pm on a work day – it’s a very low engagement time of day. If you don’t have location for your subscribers you should start earlier and if you do then it’s best to optimize for local time.

Mailbox providers like Gmail and Hotmail are very sensitive to historical engagement. What this means is this – if a recipient doesn’t open 5+ emails from a sender in a short period of time (< 2 weeks), the future emails are highly likely to go to Spam. You can test this with your own seed email accounts. The only way to override this behavior is by dragging an email out of Spam folder or labeling it as “Not Spam”. So when you reduce your email frequency you improve your chances for having the recipient open your email before they start going into Spam folder.

Sounds like a lot of work? It doesn’t need to be – you can automate many things in BigMailer. Consider defining reusable segments to use in your campaigns. To create a segment go to Lists tab and click on All Contacts list, click on Search and specify your segmentation rules, then use “Save as a Segment” button. Once a segment is created it can be edited anytime.

Here are some segmentation rule examples that may help:

  1. Recently Active – Opened/Clicked Any Campaign in the last X days or recently added Engaged Segment ExampleThen you simply match that segment in your campaign: Campaign segment match
  2. Inactive audience can be used to send less frequent campaigns with the goal of re-activating. Be careful though – re-activation campaigns may cause a higher than usual complaint rate. You could simply use the same segment you defined for active subscribers but with the rule “does not match”. Campaign segment no match

Consider your sending frequency when deciding how to define your active subscribers. If you are sending daily, perhaps you only send to those who opened in the last 3-4 days. If you send weekly, a 14-21 day timeframe would be reasonable.

Special Case: Automated RSS Campaigns

If you use a daily automated RSS campaign you could consider reducing frequency from daily to once a week for anyone who doesn’t open any campaigns for 3-4 days, using the same segmentation rules discussed above. You could create a separate RSS campaign and use a reverse rule on a segment so your audience shifts from daily to 1/week automatically and as soon as they open they will shift  back to daily campaign again. You could even take this further and setup a 3rd campaign where audience shifts with no opens in 30 days. This helps you avoid sending to inactives and avoid spam folder overall.

RSS campaigns

Example: RSS campaigns to active and inactive contacts

If you are a BigMailer customer you can reach out to us via chat to get our assistance with your deliverability. Not a BigMailer customer? Get started with a free account.