We are not receiving the email

Hello, I am having a dedicated server with Inmotionhosting. I have hosted a domain named as "www.sawan.net.in" on the above server. The problem is one of the contact with email id "[email protected]" is seding the mail to a user with email id "[email protected]". But [email protected] is not able to receive the mail. More over the user "[email protected]" receives a bounce back with the following message : ======================================================================================== 20-elite1308.inmotionhosting.com ESMTP Exim 4.87 #1 Wed, 06 Jul 2016 16:21:17 +0530 220-We do not authorize the use of this system to transport unsolicited, 220 and/or bulk e-mail. helo mailgw.eil.co.in 250 elite1308.inmotionhosting.com Hello mailgw.eil.co.in [115.112.162.131] mail from: 250 OK rcpt to: 550-Callback setup failed while verifying 550-Called: 121.241.99.13 550-Sent: MAIL FROM:<> 550-Response: 501 Syntax error: Empty email address. 550-The initial connection, or a HELO or MAIL FROM:<> command was 550-rejected. Refusing MAIL FROM:<> does not help fight spam, disregards 550-RFC requirements, and stops you from receiving standard bounce 550-messages. This host does not accept mail from domains whose servers 550-refuse bounces. 550 Sender verify failed 421 elite1308.inmotionhosting.com: SMTP command timeout - closing connection ================================================================================ We asked the user to send on another domain [email protected] and [email protected] and on those email IDs we are receiving emails from "[email protected]" , there is not problem with the domain of the sender, SPF is set OK for their domain, so something seems to be wrong with our server only. I have raised a ticket for the same as well but still the problem has not been resolved? Please do the needful at the earliest. Regards, Ripal Bhatt