Aria's comment is correct and makes me think that, in the absence of a full solution, cohost simply just did not do the current parsing that results in a mailto: link.
It's worth noting that finding out the proper URI for a Fediverse account is not a simple regex job. The network is not comprised only of Mastodon instances, and not every instance runs on a TLD, making matching an account to a URI unpredictable. For example, I run an Akkoma server on a subdomain, with an address that doesn't include the subdomain—only reliable way to know is to query .well-known webfinger resources to figure out the proper canonical URL.
Kez
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