Cannot send from account

Cannot send from account

1) Whenever I try to send a message I get the following (recipient and our domain redacted)

client@hiscompany.com NVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :74.201.84.163 is not allowed to send mail from
            ourdomain.com

2) similarly, when I try to send from a pop3 account;

3) no mail received at myaddress@ourdomain.com is forwarded to my cell phone's pop3 account anymore.

We sent a mail to support but we need help urgently. 

Thanks.