vCenter Server email notifications fail due to SMTP relay restrictions on the configured mail server.

Products

• VMware vCenter Server

Issue/Introduction

• When configuring an SMTP server in vCenter for alarm email notifications, test emails or alert notifications fail to deliver.
• ESXi and vCenter alarms do not generate notification emails.
• Network connectivity between vCenter and SMTP server over port 25 is confirmed.
• In /var/log/vmware/messages:
  • sendmail[4050465]: 59T4iIgg4050465: to=<[email protected]>, relay=[SMTP_server_IP], dsn=5.7.1, reply=550 5.7.1 Unable to relay for [email protected]
  • sendmail[4050465]: 59T4iIgg4050465: return to sender: User unknown

Environment

• VMware vCenter 7.x
• VMware vCenter 8.x
• VMware vCenter 9.x

Cause

The issue occurs because SMTP relay is not enabled for the vCenter Server on the mail server.

vCenter does not directly manage SMTP relay permissions. It relies on the configured SMTP server to send outbound mail. If the SMTP server is not configured to allow relay from the vCenter’s IP address, the server rejects the mail request with the 550 5.7.1 Unable to relay error.

Resolution

To resolve the issue, enable SMTP relay for the vCenter Server on the mail server.