Hi @IBN
Which client does your contact use? Skype or Skype for Business?
Above all, please check if the contact user's domain is allowed in Microsoft Teams Admin Center > Org-wide settings > External access.
Based on my experience, if a Teams only mode user chat with an external contact who is in Islands mode, the message will be sent to the external contact’s SFB client. And if this contact is OFFLINE, then our user will receive the error messages like you posted. Please check if this is your case.
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