Hardly possible at all. Mail addresses depend on the protocol used, the server implementation used (internal email may well dispense with domains for example in many organisations), etc. etc. And verifying whether it's an existing address at all is next to impossible anyway, even if it is syntactically correct. It would involve sending a message and checking whether an answer is received at some point (which may be weeks or months later if the owner of the address is slow to respond, and may never arrive if the owner (like me and ever more people) doesn't respond to such things which are practically only ever used to harvest email addresses for spammer and identity thieves).