Cancellation of construction contract is not confirmed

Text = mail is sufficient

If text form is sufficient, then an email is enough, yes.
However, I couldn't yet read whether the cancellation policy requires text or written form.

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P.S. I only send such things by registered mail with return receipt, hand-delivered.

A registered letter (even with return receipt) only confirms that someone has received an envelope.
Therefore, you should always have a reliable witness who can confirm what you have sent.

With return receipt it is even the worse option, because there is the possibility that the mail carrier does not find anyone and only leaves a pickup notice. If no one picks up the letter, it does not count as delivered.

Better is a registered mail with delivery confirmation because it counts as delivered upon insertion.

Fax with protocol was also good.

The safest is by messenger and in such a way that he knows the contents of the envelope and can confirm it.

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I have very important mail delivered by a bailiff.
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Well then ... there should certainly be no problems even with registered mail with return receipt or even standard letters with reputable business partners.

To be precise: The revocation is not subject to any formal requirements. It can also be declared orally. It is stated in the Building Code (355er, there are no formal requirements there).