The two exceptions in § 25(2) TDDDG are: the sole purpose is to transmit a message over a public telecommunications network, or the storage or access is strictly necessary so that a provider can deliver a digital service the user has expressly asked for.
The EDPB’s Guidelines 2/2023 (version 2, adopted on 7 October 2024) describe the case of a sender who includes a pixel to detect when the receiver reads the email. They say that delivering a pixel counts as storage on the device and that collecting the identifier it sends back counts as gaining access. HawkOpen does not assess whether an exception fits your use; ask a lawyer or your data protection authority.
What HawkOpen does: a notice in every tracked email with a link where the recipient turns tracking off, and settings to exclude addresses, domains or countries from tracking.
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.