Domain .htaccess
You need to put this in each domain's .htaccess to work.
So you need it i domain1.tld, domain2.tld, ...
The syntax of the rewrite condition is specific for each domain:
RewriteCond %{HTTP_HOST} ^mail\.domain1\.tld
RewriteCond %{HTTP_HOST} ^squirrel\.domain2\.tld
...
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