Not recieving incoming emails

Hi,

Last couple of days we are having some issue with incoming mail. It started with delays, but today people sending us mail started receiving these messages:

Subject: Warning: message 1bAYyG-0007I2-J4 delayed 24 hours

This message was created automatically by mail delivery software.
A message that you sent has not yet been delivered to one or more of its
recipients after more than 24 hours on the queue on XXXXXXXX

The message identifier is:     1bAYyG-0007I2-J4
The subject of the message is:
=?utf-8?Q?dosta_lo=C5=A1e_...XXXXXXXX=
The date of the message is:    Wed, 8 Jun 2016 10:36:17 +0200

The address to which the message has not yet been delivered is:

XXXXXXXXXX
    host mx2.zoho.com [74.201.155.220]

Could you please check what is going on and let us know as soon as possible.

Thanks