Getting "The SMTP address has no mailbox associated with it." issue even for valid users

Hello Team,

I am trying to sync Outlook meetings with liferay calendar. I am creating the connection just once. 

There are some users in liferay that do not have a valid email address. While trying to fetch the appointment for an invalid user I get the issue "The SMTP address has no mailbox associated with it." which is fine. But once this error has appeared I keep on getting the same issue even for valid users. I have logged in using admin and I am impersonating other users

When I create a new connection every time I impersonate a user, then I do not get this issue, but creating a new connection always makes the process slow and thus performance is suffered.

Is there anything that I can do about it.

Thanks,

Pratibha