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My mail server is being blocked when sending mail to a couple hosts. This is the message from bounced mails <br />
<br />
There was a SMTP communication problem with the recipient's email<br />
server. &nbsp;Please contact your system administrator.<br />
&nbsp; &nbsp; &lt;mailservername #5.5.0 smtp;550-REJECTED - MULTI-BLACKLIST -<br />
[S=3 - &nbsp;rep.mailspike.net] - &nbsp;NoHeloHost&gt;<br />
<br />
What is the cause of this particular error?
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My mail server is being blocked when sending mail to a couple hosts. This is the message from bounced mails 

There was a SMTP communication problem with the recipient's email
server.  Please contact your system administrator.
    <mailservername #5.5.0 smtp;550-REJECTED - MULTI-BLACKLIST -
[S=3 -  rep.mailspike.net] -  NoHeloHost>

What is the cause of this particular error?

REJECTED - MULTI-BLACKLIST -[S=3 -  rep.mailspike.net] -  NoHeloHost&gt;

Various techniques are used to prevent email spam (unsolicited bulk email). No technique is a complete solution to the spam problem, and each has trade-offs between incorrectly rejecting legitimate email (false positives) vs. not rejecting all spam (false negatives) - and the associated costs in time and effort. Anti-spam techniques can be broken into four broad categories: those that require actions by individuals, those that can be automated by email administrators, those that can be automated by email senders and those employed by researchers and law enforcement officials.

AntiSpam

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

REJECTED - MULTI-BLACKLIST -[S=3 - rep.mailspike.net] - NoHeloHost&gt;

REJECTED - MULTI-BLACKLIST -[S=3 - rep.mailspike.net] - NoHeloHost>