If you use Microsoft Exchange as a mail relay for other servers, and on the server from which you send mail, you have an error Relay = exchangerelay.domain.com [10.10.10.10]: 25, delay = 5.1, delays = 0.04 / 0.01 / 0.05 / 5, dsn = 5.7.1, status = bounced (host exchangerelay.domain.com [10.10.10.10] Said: 550 5.7.1 Unable …