GMX and WEB.DE describe the option for the email you are writing; their help pages name no setting that requests a receipt for every email. For T-Online, freenet or Vodafone mail, look in the options of a new email or in the provider’s help. A receipt only says that the recipient’s program reported an opening, not that the text was read.
A read receipt is a request inside your email; the recipient’s program decides whether to answer it. The standard for receipts (RFC 8098) recommends that programs ask the recipient first and send nothing by default, so a missing receipt does not mean the email was not read.
HawkOpen does not use read receipts. You can connect a GMX, WEB.DE, T-Online or other mailbox to HawkOpen with IMAP and SMTP, using the server names, ports and password from your provider (many providers ask for an app password), and send tracked emails from the HawkOpen dashboard. Only mail servers on public addresses are accepted, by default on ports 465 or 587 for SMTP and 143 or 993 for IMAP. HawkOpen is in a controlled launch, so each provider connection is verified before it is offered in production.
Every email tracked by HawkOpen ends with a short notice that it contains tracking elements, with a link where the recipient can turn tracking off. An open is an estimate, not proof that someone read the email.
Where rules that require consent apply, tracking may start only after that consent has been obtained. The link in the message lets the recipient withdraw consent later.