UPDATE: While I got the answer I needed down below, this question was later asked again in this post and received a more elaborate answer.
My magento store is hosted on a 'new' gTLD (.boutique), but Magento does not allow me to set the store email addresses with this tld.
I understand that Magento's validation runs through the Zend framework and the responsible file for validating the TLDs is 'Hostname.php' in /lib/Zend/Validate/.
However, adding all the TLDs from http://data.iana.org/TLD/tlds-alpha-by-domain.txt as suggested in this thread on the Magento forums did not solve my problem.
I thought perhaps it was an issue with Magento storing the previous Zend file in a cache, so I already tried clearing all cache files from the cache management page; this did not resolve the problem.
Am I overlooking any other arrays or such that should include all the TLDs? Should I clear the cache in another way? Anything else altogether?
Any help is much appreciated.
EDIT: I was naughty and actually edited the Zend file in /lib/Zend/Validate/Hostname.php after making a Hostname.php.bak file in the same dir. Moving the edited Hostname.php file to /app/code/local/Zend/Validate/ and restoring the original Hostname.php.bak in the Zend library as suggested by the marked answer solved the problem.