Sporadic yet frequent email issue

[lost108]

Hello,

We currently have about 200 domains on our server. Every so often, several domains will start bouncing back emails with the following error:

mail.info:Jul 14 08:26:39 srvrName postfix/virtual[20115]: 3DDA5E22FC8: to=<alex@aDomain.com>, relay=virtual, delay=0.06, delays=0.06/0/0/0, dsn=5.1.1, status=bounced (unknown user: "alex@aDomain.com")

Every single time, we check the database and see that the user in question does indeed exist. By changing the status of the domain's mail users to "change" and calling the ispCP request, the issue is immediately resolved until it occurs to another domain.

Has anyone encountered this same issue? What might be causing it? Is there a way to prevent it / manage it in real time so that our users do not miss any emails?

Thank you for your suggestions!