Maximize your email delivery this holiday season

Email delivery is a tricky subject matter, and we never want to understand it more than during periods where we’re sending mass emails. This question from my line manager on a Friday afternoon started an interesting conversation for the delivery operations and deliverability teams. My first thought (well, not my first thought – that was the witty reply of “Saturday”, and much chortling ensued) was that they must be my least favorite American imports: Black Friday and its bumbling step-sibling, Cyber Monday.

I was pretty surprised to find out that it’s actually Singles Day on November 11th, a popular shopping day for our friends in the Asia-Pacific region. This year Alibaba set the record with sales surpassing $30.8 billion in just 24 hours.

We’re not yet on board with Singles Day here in Europe, but UK retailers and consumers have embraced the aforementioned Thanksgiving weekend as an opportunity for pre-Christmas sales and bargains. This means emails. Lots and lots and LOTS of emails.

During busy periods, recipients receive so many emails that marketers have to vie for their attention. This kind of inbox exhaustion is a known challenge. But another common sentiment we hear from customers during the festive period is, “why is it taking so long to deliver my emails?”

Delays can be frustrating, especially if you’re sending time-sensitive emails for short-lived sales. So, why do delays occur and what can you do to avoid them?

Queues aren’t just a British thing

In email delivery, queuing happens in two places: on the mailserver sending out the emails and on the mailserver receiving the emails.

When sending emails, the server has to wait for an IP address to have an available outbound connection opportunity, then connect to the recipient mailserver, and finally deliver the email to that recipient server. While waiting for available IPs, the mailserver will create queues of emails which are all going to the same domain (gmail.com, hotmail.com, etc.) so that it’s ready when a connection becomes available to push as many emails through that connection as it can.

On the receiving end, the servers have the unenviable task of taking those inbound emails and making sure they end up in the correct inboxes. During this process, the emails are checked by filtering tools and algorithms to ensure they’re wanted mail. Queues will be created of emails waiting to be checked for spam, to then be allocated to the correct inbox, etc. Mailbox providers prioritize person-to-person communications (that thanksgiving update you sent to your grandma) over bulk marketing communications.

Just think of it like traffic: you have cars queuing at the barriers to exit parking lot A and get onto the road to their destination. There are only a certain number of exits and the road outside is busy, so they have to wait their turn. When they get to parking lot B, they have to queue for a security check and then queue to park on the right level and in the right space.

Email delivery gridlock

In general, when sending out emails, the more IPs you have the faster emails can be transmitted to a receiving mailserver. However, there are some pretty hefty caveats that apply on a day-to-day basis, even outside of the Black Friday/Cyber Monday and festive periods.

For example, most mailbox providers will apply various fair-usage limits on the number of inbound connections, the number of messages that can be sent per connection, the total number of messages they’ll accept per minute/hour/day, etc. These limits are often lower for IPs/domains which have poorer reputations and higher for those with better reputations, but most will have a cap — even for the best senders.

What happens when email volume is high?

It’s possible, and has happened historically, that receiving mailservers can be overwhelmed by the volume of email being sent during busy periods. When experiencing a high load, they may be more restrictive on the number of connections they’ll accept — even from IPs with excellent reputations.

When mailbox providers are telling our mailservers that they’re under high load, we automatically reduce our send rates and the number of connections we’re making. This slows down sending and lengthens queues, but it means that we’re less likely to irritate a mailbox provider to the point where they throttle the sending IPs (i.e. impose their own, even more severe, rate limits) or block them outright. Our system can detect when things are back to normal and send rates will increase again.

Even after emails have been accepted by receivers, during busy periods they may have enormous queues waiting for their filtering and allocation tools to check emails for spam and actually get them to the inbox. This is where person-to-person prioritization comes in: you might be able to send a single test email from your work Office 365 account to your personal gmail account and it’s delivered in seconds, but marketing emails from your ESP could take much longer. This is because recipients are more likely to complain to their mailbox provider about delays in receiving an email from their niece than complain about a delay in receiving a marketing email.

What you can do about email delivery

Firstly, choose a responsible ESP that understands the challenges of busy periods and has the ability to scale up during times of high load. Our deliverability operations team has been hard at work all year, expanding the number of IPs in use and bolstering our sending infrastructure.

Secondly, put yourself in the shoes of the person receiving your emails. Just spend 10 minutes in your own personal inbox, being overwhelmed by the influx of marketing emails. Have some empathy for your recipients and use targeting and segmentation to send something really interesting to people who are likely to engage with your emails.

Finally, and most importantly, be sympathetic with mailbox providers. Their goal is to provide the best experience to their users and during busy festive periods they manage queues and prioritize accordingly. Don’t leave campaigns until the last minute; instead, think about spreading your sends out over a few hours or days. Every email you send to gmail or Hotmail or Yahoo uses up some amount of processing power and then some amount of storage space – and you don’t pay for that. Be patient and learn from experiences this holiday season to inform your future strategies.


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