$string  =preg_replace('#<A HREF=(?!(MAILTO))[^>]+>[^<]+</A>#i', '', $string);

If you want it to be case insensitive

$string  =preg_replace('#<A HREF=(?!(MAILTO))[^>]+>[^<]+</A>#i', '', $string);

If the HREF attribute should be in quotes:

$string  =preg_replace('#<A HREF="(?!(MAILTO))[^>]+>[^<]+</A>#', '', $string);


the key here is the (?!(MAILTO)) which is called negative lookahead.
The expression a(?!x) means "a not followed by x". In the expression above
HREF=(?!(MAILTO)) means "HREF= not followed by MAILTO"