Diagnostic information for administrators:

 Generating server: my-exchange-svr.com

 user@domain.com server4519x.domainhosting.com Remote Server returned '550-Please turn on SMTP Authentication in your mail client. 550-my-exchange-svr.com (my-exchange-svr.com) 550-[my-exchange-ip_public]:19855 is not permitted to relay through this server without 550 authentication.'

any suggest?

which part must be changed in our Exchange Svr configuration?

Hi,

take a look here, it may be the same problem

https://social.technet.microsoft.com/forums/exchange/en-US/932093f9-cca5-40a5-a659-5d69c8d23c33/mail-rejected-because-of-550-enable-authentication-error

Best regards

Hi Noval,

Have you checked the above suggestion ? How about the result ?

Best regards,

it's server side problem,not in user,because they can send email a few moment later.without changed in server or mail client

the suspect from domain name,domain target cannot resolve because there is in one forest in our domain trusted relationship,when use public dns,it's resolve by public IP. so the issue is in dns lookup not in exchange server.