Facing server issues with iCal Plugin in OnPremise Environment

I am using iCal Plugin 6.0.0 in OnPremise Appian environment, to send an invite to Outlook.

The node gets completed successfully in the PM and even a meeting uId is generated. However, meeting invite is not received in recipients' inbox(this recipient is able to receive normal emails from the same environment).

On further investigating the tomcat logs, it looks like an issue with smtp/server errors, which is not able to send the invite.

Please share thoughts/suggestions.

Thanks in advance.

Nitika