Flowmailer Glossary

An overview of important concepts

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When an e-mail cannot be delivered, the system handling the message will, in most cases, send a message back to notify the sender. Handling bounces can be tricky. Although there are standards governing how bounce messages should be structured, many providers don't follow those rules. Others intentionally obfuscate the delivery state to throw off spammers. Flowmailer tries to recognize all those messages correctly to make sure you get reliable reports.

Bounces are classified as 'hard' or 'soft'. Both mean your message was not delivered and we will not try to deliver it anymore. In some cases, that may be a temporary problem related to the message or the recipient (for instance mailbox full). We call these softbounces; later messages might still be delivered successfully. In other cases, your message was not delivered and it is very likely future messages to the same address will not be successfully delivered as well (for instance recipient unknown). To protect your reputation and ensure future deliverability, Flowmailer will add this address to your accounts filter list so future messages will be filtered before delivery.


DomainKeys Identified Mail (DKIM) is a way of cryptografically signing emails, invisible to a regular recipient. This allows receiving mailservers to verify the legitimacy of a message by comparing such a signature to information published by the sender domain. Basically this means a domain owner can vouch for an e-mail sent on its behalf. Flowmailer DKIM signs all messages by default and allows customers to sign using their own domain with Sender Identities to maximize your deliverability.


DMARC stands for Domain-based Message Authentication, Reporting and Conformance. The idea behind DMARC is that organizational owners can combine a DMARC policy record in DNS with SPF and DKIM to authenticate their domain in the From address, to help prevent phishing. Flowmailer supports DMARC using Sender Identities. Please let us know if you need help setting this up!

Feedback loop

Most large e-mail providers offer Feedbackloop programs. High volume senders such as Flowmailer can receive reports about recipients clicking the 'spam' button for a message (sometimes marking the sender as unwanted). This allows us to monitor the behavior of our customers and detect possible configuration or security issues. In most cases subsequent messages will be sent to the recipients spam folder or will not be delivered at all. This basically means that to ensure deliverability to other recipients on the same domain, we need to filter messages to recipients that hit the spam button.

However, we also noticed some recipients marking messages as unwanted, not expecting (and wanting!) to basically block that sender entirely. In these cases, we allow customers to remove that individual recipient from the filter list to try and reach that mailbox again. A good bit of advice is then to ask your recipients to move your next message (that will most likely be placed in their spam folder) to their inbox. This is a strong signal to their provider that they want to receive your messages in the future.


Flows represent the different type of messages you send to your recipients. Each Flow has its own reporting and allows you to configure Flowsteps for configuring how to handle each message.


Once a message gets assigned to a Flow, Flowsteps can be used to make changes to the message before delivering it to the recipient. This allows you for instance to apply templates, add tracking or archive the message.


Virtually all e-mails in the world are being sent via the SMTP protocol. This protocol ensures all mailservers speak the same language so that they can exchange messages. Flowmailer can accept messages using (authenticated) SMTP, so your software can send messages in the same way as when using a regular mailserver.


Any application that will be sending messages needs to be configured as a Source (also when sending via the REST API). All Sources have their own credentials and optional IP limit. This allows you to safely allow systems to send messages on your behalf, without granting that system or its administrator access to your Flowmailer dashboard.


Templates allow you to make any message look great, by selecting them as a Flowstep. We also offer design and coding services if you're not a HTML for e-mail wizard yourself. Flowmailer uses the well documented Freemarker template engine to allow for virtually any intelligent template behavior. Flowmailer adds several additional functions for your e-mails, for instance to easily use content from the original message and include a 'view online' link.

Two-factor authentication

Ever had malware or spyware on your computer? It happens to the best of us. What if some malicious browser 'toolbar' steals your Flowmailer login? The attacker would then potentially have access to all the e-mails you can access via the Flowmailer dashboard. But don't panic! Enabling two-factor authentication for your Flowmailer login means you need an additional one-time code to access the account. This code gets sent to your mailbox which only you should be able to access. This extra layer of security helps to keep your data safe, even when your credentials get stolen.