This message is used only in logging as far as i can see in the code.
You have turn on Joomla Debugging (see here
docs.joomla.org/Using_JLog
) and you have to set the debug plugin to log almost everything. Then change an email adress to a wrong adress and book an event this user.
I have used your PLG_JEM_MAILER_LOG_SEND_ERROR in my german language file and it worked
from the log file:
2020-05-12T21:07:58+00:00 WARNING ::1 jem Error sending email to
This email address is being protected from spambots. You need JavaScript enabled to view it. : SMTP-Fehler! Die folgenden Empfänger sind nicht korrekt:
This email address is being protected from spambots. You need JavaScript enabled to view it.: <
This email address is being protected from spambots. You need JavaScript enabled to view it.>: Recipient address rejected: User unknown in virtual mailbox table
HTH