Gmail is blocking emails from Exchange Server

Hello Peers,

I have a small installation of Exchange Server 2013. I am the only user who is using it and really low number of emails. Hardly 10 in a month. Everything was working fine but suddenly gmail stopped accepting emails from my exchange server. I am getting following bounce mail,

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mx.google.com rejected your message to the following email addresses:

------@xpresservers.com (------@xpresservers.com)

mx.google.com gave this error:

[2002:cc0c:ef9a::cc0c:ef9a 12] Our system has detected that this message is likely unsolicited mail. To reduce the amount of spam sent to Gmail, this message has been blocked. Please visit http://support.google.com/mail/bin/answer.py?hl=en&answer=188131 for more information. f8si9784821vcn.66 - gsmtp

Diagnostic information for administrators:

Generating server: mail.xpresservers.pk

...