Network I/O error when sending data to mx.zoho.com

Hi,

I have configured my work e-mail to deliver all messages to my zoho account. However, it seems that for relatively big messages (~ > 3MB), the delivery fails with error:

Network I/O error 10054 (0X00002746) encountered when sending message data to mx.zoho.com.

And

Network I/O error 10054 (0X00002746) encountered when sending message data to mx2.zoho.com.

I also get from my work's e-mail server a delivery failure message. A snippet of this message:

Reporting-MTA: dns; srv-xxxxxxxxxxxxxxx
Received-From-MTA: dns; xxxxxxxx (unverified [10.64.65.21])
Arrival-Date: Fri, 2 Jun 2017 14:10:59 +0300

Final-Recipient: rfc822; xxxxx@my.domain
Action: failed
Status: 5.4.7 (Permanent failure - routing/network: delivery time expired)
Remote-MTA: dns; mx2.zoho.com

Could you provide any insight?