las funciones getmxrr() y checkdnsrr() y aqui te dejo un ejemplo de como verificar si un servidor de correos esta presente para un dominio especifico :
Código PHP:
<?
$email = "[email protected]";
print("Checking: $email<br>");
if (eregi("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)+$", $email)) {
print("Format Test: PASSED<br>");
print("Online host verification Test...<br><br>");
print("MX Records for: $email<br>");
list($alias, $domain) = split("@", $email);
if (checkdnsrr($domain, "MX")) {
getmxrr($domain, $mxhosts);
foreach($mxhosts as $mxKey => $mxValue){
print(" $mxValue<br>");
}
print("Online host verification Test: PASSED<br><br>");
print("Email Status: VALID");
} else {
print(" No records found.<br>");
print("Online host verification Test: FAILED<br><br>");
print("Email Status: INVALID");
}
} else {
print("Format Test: FAILED<br><br>");
print("Invalid email address provided.<br><br>");
print("Email Status: INVALID");
}
?> cya