We updated to the latest wordpress version and the latest newsletter plugin recently.
It has been a long time since we last sent an e-mail to our members (we are a physical non-profit club and rarely send out informational e-mails to our real life members).
I noticed the system says all e-mails have been delivered but nobody received the e-mail.
I decided to send a test e-mail instead and got the following error message:
call_user_func_array() expects parameter 1 to be a valid callback, non-static method AKTT::admin_enqueue_scripts() should not be called statically
The screen says there is a log message to check on the status page but there is no new log file there.
Our current PHP version is 5.6.40
Our WordPress version is 5.5.1
Our Newsletter version is 6.8.7
Anyway, that error could not be the problem you face about mail delivery. That error may block the email deliver or may not. You should even ask the provider if there are blocks on their mailing system.