It would be great if email delivery happened instantly. However, this just isn’t the case. Instead, emails often have to pass through multiple servers before reaching their final destination, and several factors cause these delays.

This post explains why bulk emails might be delayed and how developers of Email Service Providers (ESPs) can solve these issues.

What causes delays on the Sender’s end?

When sending bulk emails, there are a few common culprits that can slow things down. Here are a few reasons your emails might be held up:

1. Slow or overloaded mail servers

Mail servers can become overloaded due to high volume, spam filtering, or other tasks, slowing down email processing and delivery. Implementing load balancing - a process of managing or allocating traffic over several servers – and optimizing server performance can help manage this issue.

2. Reputation-based deferrals

Sometimes, mailbox providers hit the pause button on your emails if your IP has some reputation issues, which can happen if you’re sending too many emails at once. They might respond with 4xx errors like “rate limited.” Your system will try to send these emails for up to 72 hours, but if they don’t break through, they’ll be discarded. 

Monitoring and managing IP reputation is crucial to minimize deferrals.

3. Connection and volume limits

Mail providers and Internet Service Providers (ISPs) have strict rules about how many connections and messages they’ll accept at once. Pushing these limits causes your emails to end up in a queue, waiting to be delivered. Staying within these guidelines and managing your send rates can keep your emails from getting stuck in line.

4. Pre-Send processing delays

Before you even hit “send,” there could be technical hiccups. Complicated segments or huge lists can slow things down. Plus, if you’re sending a campaign for the first time, it might go through extra checks to make sure it’s all legit. Streamlining these processes can speed things up.

What causes delays on the Receiver’s side?

Once your emails are out the door, they can still face some hurdles on their way to the recipient’s inbox. Here’s a look at what might be causing delays on the receiving end:

1. Slow or overloaded recipient servers

Just like your servers, the ones on the receiving end can get bogged down by too much incoming mail. When that happens, your emails might be delayed. Knowing this can help you set the right expectations with your clients or users.

2. Full mailbox

If a recipient’s mailbox is full, your email won’t get through until there’s some room. This can be more common during vacation seasons when people aren’t checking their inboxes as often. Good bounce handling practices can help you clean up these addresses and keep your emails moving.

3. Anti-Virus/Spam screening programs

These programs scan incoming emails for potential threats, which can slow things down, especially if your emails have large attachments. Making sure your emails can easily pass through these filters can help reduce delays.

4. Send/Receive cycles

Some email providers only send and receive emails at certain intervals—anywhere from a few seconds to 15 minutes. This can also cause bulk email sending delays, so it’s worth letting your users know this might happen.

Troubleshooting reputation-based deferrals

If your emails are getting delayed due to reputation issues, it’s important to dig into the details and identify what’s going wrong. Here’s how to troubleshoot and get your emails back on track.

Identifying the issue

Emails can be delayed if the sender’s IP address or domain has a shaky reputation. This might happen if you’re using a new IP with no track record or if your metrics aren’t looking so good.

Gathering data

Take a look at your Simple Mail Transfer Protocol (SMTP) logs to see what’s going on. These logs can tell you why emails are getting deferred, including bounce codes and information about the IPs and domains involved.

Key questions to ask

• Are all emails being deferred, or just some?
• Is the delay happening all the time or just occasionally?
• Are you using a new IP address or domain? 
• Is the problem happening with a specific domain or provider?

Steps to fix deferrals

• Warm-Up New IP Addresses or Domains: Start by sending a small number of emails and gradually increase the volume to build up a positive reputation.
• Follow Best Practices: Keep your lists clean and follow the guidelines provided by mailbox providers.
• Analyze Data: Keep an eye on complaint rates, volume spikes, and engagement metrics to spot any patterns. Regularly clean your lists of people who never open or click.

Real-world example

Imagine your ESP has been running like clockwork for years, but suddenly clients are complaining about delays. Some emails show up instantly, while others take hours. This inconsistency might be a sign that recipient mail servers are throttling your IP addresses or something else is slowing things down.

Possible solutions

• Retry Notifications: Set up your system to send alerts if an email has to be retried.
• Buffer Handling: Understand that recipient servers might hold onto emails for a while before delivering them.
• Throttling: Check if your IPs are being throttled by recipient servers.

If the problem persists, it might be time to open a support case with your provider. Make sure to include detailed logs, like the from and to email addresses, timestamps, and message IDs, to help them pinpoint the issue.

Understanding the ins and outs of bulk email sending delays—from email server overload to reputation issues and receiving server bottlenecks—is key for ESP developers. By sticking to best practices and collaborating closely with inbox providers, you can improve your bulk email deliverability and keep your communications running smoothly.

You can also reach out to us anytime if you have questions or need someone to walk you through this information.

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