Can't receive when upgrade mail box

Can't receive when upgrade mail box

Hello Zoho Team,

Please check for my company mail box: Zoho ID: 57408801

Web point MX record to mx.zoho.com and mx2.zoho.com
http://mxtoolbox.com/SuperTool.aspx?action=mx%3aamber-corporation.com&run=toolpage

But we can't receive when we upgrade mail box more than 10 users.

Thanks Zoho Team.