Try something like:
Let ( [ len = Length ( Mail ) ; at= Position ( Mail ; "@" ; 1 ; 1 ) ; lastDot = Position ( Mail ; "." ; len ; -1 ) ; domain = Right ( Mail ; len - lastDot ) ] ; at and at < lastDot and ( Length ( domain ) = 3 or domain = "tv" ) )
This could be a calculation field (result is Number), and you would search for 0 in it to find records that do not conform.
Note that there's a LOT more to validating e-mails - see for example: