Not sure, but you said that all your sub domains, including your main domain, are affected, which indicates that you are hosting other things beside Mailcow. Maybe one of the other sites or services have triggered the message…
Also, I’m not an expert on this, but the specific message on your screenshot does not refer to malware (otherwise you would get a different message), but to malicious activity like phishing etc… So there is a fair chance that the Google algorithm has made a mistake, i.e. it could be a false positive. If that’s the case, you could request a review from Google to have your site(s) unflagged… https://web.dev/request-a-review/
