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Symptom

Exim returns an “unroutable address” error when attempting to relay mail through MailChannels.

Cause

The error is caused by a missing space in the route_list directive in the Exim configuration. Without the space, Exim cannot parse the destination correctly.

Fix

In WHM’s Exim Advanced Editor, locate the send_via_mailchannels router in the ROUTERSTART section and check the route_list line. Incorrect — missing space:
Correct — space between * and the hostname:
The space between * and smtp.mailchannels.net is required. The * means “route all destinations” and must be separated from the smarthost address by a space. Save the configuration and restart Exim: