Q1. Do you get similar results?

Kinda sorta , see below

Q2. Why am I getting inconsistent results?

Networks are networks, they're inconsistent by nature

Or maybe old module needs TLC? Bug #106606 for Mail-CheckUser: Missing code checks in check_user_on_host

I'm guessing probably a timeout occurs or the domain check is skipped the times you get validation

Q3. Can you suggest a better way of doing this check? (Basically I want to at least check the syntax of the address and that the domain exists, and I want a result within a second.)

I might give Email::Valid a try 

#!/usr/bin/perl --
BEGIN{$Mail::CheckUser::Skip_SMTP_Checks = 1};
use strict;
use warnings;
use Mail::CheckUser qw/ check_email last_check  /;
use Email::Valid;
for (qw/ a@b.com a@abczz123ffz.com /){
    for my $i (0..9){
        print "$_ -> ";
        my $tt = time;
#~         Alpha($_);
        Omega($_);
        print " ", time-$tt, "\n";
    }
}
sub Omega {
    my $ev = Email::Valid->new( -mxcheck => 1, );
    if ( $ev->address( "$_" ) ){
        print "OK";
    }else{
        print "NOT OK ", $ev->details ;
    }
}
sub Alpha {
    if ( check_email($_)){
        print "OK"
    }else{
        print last_check()->{reason};
    }
}
__END__
a@b.com -> DNS failure: NXDOMAIN 0.293894052505493
a@b.com -> DNS failure: NXDOMAIN 0.206084966659546
a@b.com -> DNS failure: NXDOMAIN 0.219376087188721
a@b.com -> DNS failure: NXDOMAIN 0.218585014343262
a@b.com -> DNS failure: NXDOMAIN 0.249444961547852
a@b.com -> DNS failure: NXDOMAIN 0.250465869903564
a@b.com -> DNS failure: NXDOMAIN 0.249492883682251
a@b.com -> DNS failure: NXDOMAIN 0.249480009078979
a@b.com -> DNS failure: NXDOMAIN 0.249181985855103
a@b.com -> DNS failure: NXDOMAIN 0.251276016235352
a@abczz123ffz.com -> DNS failure: NXDOMAIN 0.251024007797241
a@abczz123ffz.com -> DNS failure: NXDOMAIN 0.248931884765625
a@abczz123ffz.com -> DNS failure: NXDOMAIN 0.250422954559326
a@abczz123ffz.com -> DNS failure: NXDOMAIN 0.249350070953369
a@abczz123ffz.com -> DNS failure: NXDOMAIN 0.250164985656738
a@abczz123ffz.com -> DNS failure: NXDOMAIN 0.251390933990479
a@abczz123ffz.com -> DNS failure: NXDOMAIN 0.248254060745239
a@abczz123ffz.com -> DNS failure: NXDOMAIN 0.281542062759399
a@abczz123ffz.com -> DNS failure: NXDOMAIN 0.240454912185669
a@abczz123ffz.com -> DNS failure: NXDOMAIN 0.254595994949341
a@b.com -> NOT OK mxcheck 0.291569948196411
a@b.com -> NOT OK mxcheck 0.208410024642944
a@b.com -> NOT OK mxcheck 0.219083070755005
a@b.com -> NOT OK mxcheck 0.218981981277466
a@b.com -> NOT OK mxcheck 0.24937891960144
a@b.com -> NOT OK mxcheck 0.249850988388062
a@b.com -> NOT OK mxcheck 0.328232049942017
a@b.com -> NOT OK mxcheck 0.250256061553955
a@b.com -> NOT OK mxcheck 0.265328884124756
a@b.com -> NOT OK mxcheck 0.247688055038452
a@abczz123ffz.com -> NOT OK mxcheck 0.392891883850098
a@abczz123ffz.com -> NOT OK mxcheck 0.250204086303711
a@abczz123ffz.com -> NOT OK mxcheck 0.249541044235229
a@abczz123ffz.com -> NOT OK mxcheck 0.453320026397705
a@abczz123ffz.com -> NOT OK mxcheck 0.218268156051636
a@abczz123ffz.com -> NOT OK mxcheck 0.218549966812134
a@abczz123ffz.com -> NOT OK mxcheck 0.250627994537354
a@abczz123ffz.com -> NOT OK mxcheck 0.312192916870117
a@abczz123ffz.com -> NOT OK mxcheck 0.234641075134277
a@abczz123ffz.com -> NOT OK mxcheck 0.250532865524292

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In reply to Re: Mail::CheckUser inconsistent results by Anonymous Monk
in thread Mail::CheckUser inconsistent results by tel2

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