Notes user received a mail from external system.
The Notes user was working mail rule on mail database to forward the mail to other internet address.
Even if the address format in From field on external system mail was not correct, the mail was forwarded to the Domino server where the SMTP task was running.
And then, this was the first time the following error was output on the Domino server where the SMTP task was running.
"Invalid Internet address specified."
This error should be output by the Domino server that received the mail in the first place.