Email Deliverability


How we get your emails to the inbox

Discussing three factors of deliverability that matter to us

Richard van Looijen

Managing Director



Making your emails hit the inbox is undoubtedly an art. The closer you want to get to 100% deliverability, the more technical skills are required. To ensure that the millions of emails we deliver every day hit the inbox in time, we take various technical measures. The result is a high delivery speed and inbox placement. On average we get 99,8% of emails to the inbox within a second. How?

Published in:

Mar 2022

By: Richard van Looijen

To keep inbox placement and delivery speed of all our emails high, Flowmailer is constantly working on:

  • Mail Server Optimization;
  • Enforcing SPF, DKIM, and DMARC;
  • Reputation Management.

Let's go through them shortly.

Mail Server Optimization

Flowmailer MTA

To get millions of emails in the inbox daily, we've built our very own Mail Transfer Agent (the FMTA). As you might know, every email platform needs an MTA, though this is often outsourced to a third party. The FMTA is built and managed by our experts, allowing us to continuously optimize according to the needs of our platform.

Our MTA has unique features, including a queue per receiving mail server to streamline large numbers of emails. Its clustered setup ensures that server outages can be tolerated without loss of functionality. Together with technical issues such as own IP addresses, correct DNS and Reverse DNS records and authentication, Flowmailer can guarantee high delivery ratios.

Automatic routing

Many receiving mail servers have deliberate limits on the speed at which e-mail can be delivered, often per sending IP. Sending these servers too many emails too fast will result in delays or even hard bounces, since the receiving mail server will classify these messages as spam. Our FMTA knows which email servers have such limites and adjusts its behavior accordingly (automatically and manually): e.g. by choosing the optimal number of simultaneous connections, messages per connection and messages per hour.

SPF, DKIM & DMARC support

In recent years, several initiatives have emerged to determine which servers are allowed to send email on behalf of which domain by means of DNS records and cryptographic signing. These techniques often require a specific implementation in the sending platform to support this. If this authorization is done correctly, the chance of being marked as spam is much lower.

In our onboarding of new Flowmailer users, we help set up SPF and DKIM authorization to allow sending domains to email securely.

Read more about email authentication

Reputation Management


An important aspect of email delivery, especially to large recipients, is that a sender does not continue to send emails to addresses that have previously caused a "hard bounce". Flowmailer has an extensive recognition mechanism to categorize return messages as clever as possible. The 'rules' of bounce management change constantly, which is why we keep a close eye on them.


At Flowmailer, we spend a lot of time analyzing the responses to e-mailings from our customers. If a recipient files a spam complaint, we handle it with the utmost care. In addition, we look for potential red flags: e.g. email campaigns with a remarkably high unsubscribe rate. We find errors in the opt-in registration processes of our clients or the lack of a proper unsubscribe option. By subsequently providing the customer with advice on solutions, we keep the recipients happy and prevent future delivery problems.

Feedback loops

Some large email recipients (like Gmail or Yahoo) report back to Flowmailer when an email sent by us is marked as spam. We then ensure that our client can no longer email this recipient (automatic opt-out). In return, our messages are rated more leniently in their spam checks.

Blacklist/blocklist monitoring

All over the world, blacklists (or: blocklists) are maintained to help eliminate spam. Examples are Spamcop and SORBS. Many receiving mail servers check these lists to block spammers. These lists are a risk; when (rightly or wrongly) mentioned on such a list, the delivery of many domains comes to a complete halt. Flowmailer therefore uses a set of IP addresses that, in case of blacklisting, can be disabled until the blacklisting is resolved. We notice this by continuous automatic monitoring on many of these lists.

Active management

The email landscape is constantly evolving and Flowmailer, too, is regularly confronted with changes that need to be anticipated as efficiently as possible. For example, what happens to the email addresses of a provider that stops its service (e.g. In addition to the various automatic monitoring processes, we therefore check on a daily basis whether there are any bottlenecks in the delivery. Delivery therefore requires continuous attention and is, in our view, a profession in itself.


How we get your emails to the inbox

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Richard van Looijen

Managing Director



One of the founders and currently Managing Director of Flowmailer, Richard is a tech geek by heart and loves to spread his ideas and views on how email works and should work.

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