--- title: "Rechnungskit chooses AhaSend" description: "Rechnungskit turns payments into invoices for German online businesses. It plugs into nearly all popular shop systems and payment providers. Every paid order becomes a ZUGFeRD e-invoice: a normal PDF for people, with the invoice data embedded as XML for the accounting software on the other side. Refunds become cancellation invoices, payouts are matched to the orders behind them, and at the end of the month the tax advisor gets a DATEV export instead of a folder of PDFs. Everything runs on servers in Germany." url: https://ahasend.com/blog/rechnungskit-chooses-ahasend markdown_url: https://ahasend.com/blog/rechnungskit-chooses-ahasend.md lang: en type: blog_post published: 2026-11-03 updated: 2026-11-03 author: "Mark Kraakman" category: "Case Studies" image: https://cms.ahasend.com/sites/default/files/2026-11/rechnungskit.png --- # Rechnungskit chooses AhaSend _By Mark Kraakman, November 3, 2026, in Case Studies_ ![Rechnungskit chooses AhaSend for invoices sent in its customers' names](https://cms.ahasend.com/sites/default/files/2026-11/rechnungskit.png) [Rechnungskit](https://rechnungskit.de/en) turns payments into invoices for German online businesses. It plugs into nearly all popular shop systems and payment providers. Every paid order becomes a ZUGFeRD e-invoice: a normal PDF for people, with the invoice data embedded as XML for the accounting software on the other side. Refunds become cancellation invoices, payouts are matched to the orders behind them, and at the end of the month the tax advisor gets a DATEV export instead of a folder of PDFs. Everything runs on servers in Germany. Its customers range from small D2C brands and coffee roasters to SaaS companies processing millions of euros a year. What they have in common, says founder Christian Häfner, is that "most of them are good at selling and lose hours every month on invoices and the hand-off to their tax advisor." That makes email unusually load-bearing for Rechnungskit. Every invoice it produces goes out by email, in the merchant's name, to the merchant's customer. When one does not arrive, the buyer asks the merchant, and the merchant asks Rechnungskit. ## The problem Before AhaSend, Rechnungskit sent its mail through Brevo's SMTP relay. Delivery worked. Visibility did not. "Once a mail had left our server we couldn't see what happened to it," says Christian. "If a buyer had mistyped their address, the invoice bounced and nobody noticed, neither we nor the merchant. For a product that sends invoices in other companies' names, I didn't want to live with that." The damage from a silent bounce is not just a missing PDF. Business buyers need the invoice for their own bookkeeping, so one that never arrives creates work on both sides, and the blame lands on Rechnungskit. "If an invoice never arrives and nobody can say why, we look unreliable even when the buyer mistyped their address." There was a second, quieter constraint. Rechnungskit processes data on behalf of its customers, so every subprocessor appears in its data processing agreement, and merchants and their tax advisors read that list. An email provider sees the names and addresses of the merchants' customers plus every invoice as an attachment. "Many of our customers are explicitly looking for tools without US subprocessors. An email provider sees every invoice we send, so a US one was never an option for us." ## Why AhaSend Christian went looking for an HTTP API with a short, specific list of requirements: - A message ID per mail, to store next to the invoice - Signed webhooks for deliveries and bounces - Idempotency keys, so a retried job can never send a buyer the same invoice twice - A sandbox to check credentials without sending anything - A European company that keeps data only briefly and does not track opens or clicks "AhaSend had all of that," he says. "I created the account on a Thursday morning, and our production mail was running through it the same day." The whole migration fit into that one day. AhaSend became the provider in Rechnungskit's shared mailer, the webhook went live, and bounced invoice mails started creating tasks. Because Rechnungskit sends from a verified subdomain and keeps the merchant's address as Reply-To, its customers did not have to change anything. Brevo stayed on as a fallback for a few days, then was removed, and the data processing agreement and privacy policy were updated. On the AhaSend side, message content is deleted after one day and open and click tracking is switched off for the whole account. For a company whose subprocessor list gets read, that is the part that matters. ## One provider, four jobs Most SaaS companies split their email: a transactional API for the app, a mailbox provider for the people. Two weeks after the first migration, Rechnungskit had moved the rest of its mail over too, including the company mailboxes. Today AhaSend sits in four places: 1. **Invoices sent for merchants.** By default every invoice goes to the buyer from email.rechnungskit.de, in the merchant's name, with the merchant's address as Reply-To. Payment reminders, cancellations, logins and notifications go out the same way. 2. **Merchants' own AhaSend accounts.** Merchants who want invoices to come from their own domain connect their own AhaSend account in Rechnungskit's settings, and Rechnungskit sends through that account instead. A [help page](https://rechnungskit.de/en/docs/features/ahasend) walks them through it. 3. **Every mail to and from the company.** Support, partner and personal mail for every address on rechnungskit.de. Incoming mail arrives at AhaSend, which hands it to Rechnungskit's own CRM. 4. **The newsletter**, on its own domain news.rechnungskit.de, so a campaign can never affect invoice delivery. "AhaSend could cover all of it, and one provider keeps things simple," says Christian. "With a mailbox provider next to a transactional API, all of that mail would sit in a second system with its own DPA, and we'd have to sync it into the CRM." ## The inbox became a webhook The third item on that list is the unusual one. Until the end of October, mail to rechnungskit.de landed in Google Workspace. Now there is no mailbox at all. Every incoming message hits an AhaSend inbound route, which posts it to a webhook in Rechnungskit's own app. The app stores it in its own database in Germany and files it under the right contact and company, next to their account, chat messages, call notes and earlier conversations. Support requests, partner mail and Christian's personal mail end up in one work list, and he answers from there. Replies go back out through the AhaSend API with the right threading headers, so they show up in the same conversation on the other side. "The CRM is the part I'm happiest with," says Christian. "We built it ourselves, and AhaSend's inbound routing is what makes it work. We also decide ourselves how long mail is kept." It is a small team's version of a problem every B2B company has: customer conversations scattered across a mailbox, a support tool and a CRM, synced badly or not at all. Rechnungskit solved it by removing the mailbox. ## How it's working out "Very well so far," says Christian. "Mail to Gmail, GMX, web.de and T-Online usually arrives within seconds of AhaSend accepting it. We haven't had a single outage or delivery problem since the switch." The bounces did not disappear. They became visible. Almost all invoice mails are delivered, and the few that come back are down to the recipient: mailboxes that do not exist, typos in the domain, full inboxes. Each of those now turns into a task for the merchant, with the reason and a button to resend to a corrected address. "Before the switch, nobody would have found out about them. Now we can tell the merchant that it bounced, why, and where to send it instead." On the integration side, Rechnungskit built the AhaSend API in twice: once for its own sending, once for merchants who bring their own account. - The ZUGFeRD PDF (a PDF/A-3 with the XML embedded) goes along as a normal attachment over plain JSON and HTTPS. - Webhooks follow the Standard Webhooks spec, so verifying signatures on the raw body took a few lines. AhaSend's message ID is stored with every invoice mail, and delivered, deferred and bounced events are matched back to it. - A merchant who connects their own account hands over an API key scoped to the messages:send permission for their domain only. Rechnungskit stores it encrypted. If AhaSend refuses a send, for example because the domain is not verified yet, the merchant sees a plain-language error with a link to the help page instead of a raw API response. "The integration was straightforward," says Christian. "It's plain JSON over HTTPS." ## What stood out Three things Christian did not expect. **Sandbox mode.** It checks the API key, its permissions and the sending domain like a real send, but delivers nothing and costs nothing. Rechnungskit built its "test connection" button for merchants on top of it. **How much of the setup lives in the API.** Domains and their DNS status, inbound routes, data retention. Rechnungskit checks its domains every hour and creates its own inbound routes. "The only manual step left for me was DNS. The setup runs itself, so I don't spend time in dashboards." **The MCP server.** "While I was writing these answers, my coding assistant pulled our delivery and bounce stats straight from AhaSend, so I could check how things are going without opening the dashboard." The newsletter is next. It runs on news.rechnungskit.de through AhaSend, with sign-up and sending built into Rechnungskit itself: double opt-in, one-click unsubscribe, no tracking, and a staged send that goes to a small part of the list first and pauses on its own if too many addresses bounce. It will cover what is new in Rechnungskit plus practical notes on e-invoicing, GoBD and DATEV for businesses that sell online. ## Would Rechnungskit recommend AhaSend? It already does, inside the product. When merchants choose how their invoices are sent, AhaSend is the option marked as recommended. "For a German SaaS the main reasons are that AhaSend is a European company with the data in the EU, and that the API is something a developer can wire up in an afternoon," says Christian. "The 1,000 free mails a month also help while you're small." ## At a glance - **Company:** Rechnungskit, Germany. Turns orders and payment-provider data into ZUGFeRD e-invoices, GoBD archive and DATEV exports for online businesses. - **Before:** Brevo SMTP relay for the app, Google Workspace for the team. No visibility after send. - **Switched:** October 2026, in a single day. Company mail followed two weeks later. - **Uses AhaSend for:** invoices and reminders sent in merchants' names, merchants' own connected accounts, all inbound and outbound company mail routed into a home-built CRM, and a newsletter on a separate domain. - **What changed:** bounced invoices now surface as tasks for the merchant with the reason and a resend button. Zero outages since the switch. Google Workspace removed. - **Why it fit:** EU company, one-day content retention, no open or click tracking, Standard Webhooks, idempotency keys, sandbox mode, domains and inbound routes managed through the API. Sending something your customers' customers have to receive? Start for free on AhaSend → https://ahasend.com and see every bounce.