Cannot add invitee to calendar events

Anyone seen this issue?

<""@alexecollins.com>: host mail.alexecollins.com[private/dovecot-lmtp] said:
   550 5.1.1 <@alexecollins.com> User doesn't exist: @alexecollins.com (in
   reply to RCPT TO command)
Reporting-MTA: dns; mail.alexecollins.com
X-Postfix-Queue-ID: C04D9FBBAB
X-Postfix-Sender: rfc822; alex@alexecollins.com
Arrival-Date: Thu,  2 Feb 2017 11:05:16 +0000 (UTC)

Final-Recipient: rfc822; ""@alexecollins.com
Original-Recipient: rfc822;@alexecollins.com
Action: failed
Status: 5.1.1
Remote-MTA: dns; mail.alexecollins.com
Diagnostic-Code: smtp; 550 5.1.1 <@alexecollins.com> User doesn't exist:
   @alexecollins.com
From: alex @alexecollins.com
Subject: Cancelled: THE REDACTED
Date: 2 February 2017 at 11:05:16 GMT
To: REDACTED@REDACTED
Reply-To: alex <REDACTED@alexecollins.com>

Sorted. There was a space prefixing the username.