Why does it take sometimes hours for an email to get delievered?

This is not MaxBulk Mailer fault. MaxBulk Mailer delivers your messages directly to your ISP (Internet Service Provider) e-mail server. During that process it marks messages as sent *only* when the server responds with an acknowledgment and a delivery receipt so at that point the server is in charge of contacting each recipient SMTP server in order to deliver the message. This is how SMTP protocol works. Usually your SMTP server will try to contact each recipient e-mail server as soon as possible. If a hard error occurs a bounce is automatically generated (e.g. domain doesn't exist). If the delivery can't be done for any other reason it will try to connect during 5 days. If after 5 days the delivery is still not possible a hard bounce is generated.

Usually messages are delivered almost immediately however if a server fails the message can be delayed 1 hour, 2 hours... 1 day... until 5 days.

You can find out where a delay comes from. Just tell your recipient to send you the full message source in order to look at headers. You will see many 'Received:' headers. The upmost is the last one so you have to read them starting from last one. For example a message may include the headers below:

Received: from vmmr4.verisignmail.com (vmmr4.verisignmail.com [10.166.0.143])
by ms9.verisignmail.com (Mirapoint Messaging Server MOS 3.2.2-GA)
with ESMTP id AEC72391;
Wed, 15 Oct 2003 12:12:27 -0400 (EDT)
Received: from vmbm4.verisignmail.com (vmbmnat.verisignmail.com [216.168.230.170])
by vmmr4.verisignmail.com (Mirapoint Messaging Server MOS 3.2.2-GA)
with ESMTP id QFT66995;
Wed, 15 Oct 2003 12:12:26 -0400 (EDT)
Received: from spidey.easydns.com (smtp.easyDNS.com [216.220.40.247])
by vmbm4.verisignmail.com (8.12.10/8.12.10) with ESMTP id h9FGCNiM012836
for <[email protected]>; Wed, 15 Oct 2003 12:12:24 -0400 (EDT)
Received: from ams004.ftl.affinity.com (lvs00-fl-n04.valueweb.net [216.219.253.138])
by spidey.easydns.com (Postfix) with ESMTP id 3A22173C2F
for <[email protected]>; Wed, 15 Oct 2003 12:07:43 -0400 (EDT)
Received: from [192.168.0.2] ([12.236.72.133]) by ams.ftl.affinity.com with ESMTP id <230100-6865>; Wed, 15 Oct 2003 12:06:36 -0400

if you remove everything but dates you get:

Received: Wed, 15 Oct 2003 12:12:27 -0400 (EDT)
Received: Wed, 15 Oct 2003 12:12:26 -0400 (EDT)
Received: Wed, 15 Oct 2003 12:12:24 -0400 (EDT)
Received: Wed, 15 Oct 2003 12:07:43 -0400 (EDT)
Received: Wed, 15 Oct 2003 12:06:36 -0400

So this message took about 6 minutes to reach its final destination and about 1 minute for the ISP server to reach first relay server.