Unconfirmed Accounts - Emails Not Sent

So far this week I have had three accounts become unconfirmed with users saying they did not receive the emails. I have resent them to all, verified their addresses and asked them to check their spam folders. One the three accounts was one belonging to me to test with.

What else can I do to troubleshoot to see if they are leaving my system? Any ideas? Is there somewhere in the Linux OS I can check this myself or should I contact my hosting provider?

Thanks in advance.

John

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