Timeout while sending A/R Invoices/Statements via Email

Sage 300 2019 PU7

While sending A/R Invoices/Statements via Email, the system sends some e.g. 50-90 and returns the Error below;

"Description: Mail delivery failed. Use the Email tab on the Company profile screen to send a test email and confirm that all settings are correct."

Without changing any settings we restart the process from where it stopped and it sends another 50-90 then returns the same Error.

Finding out where it's stopped and restarting again and again is very demoralizing for over 600 Invoices/Statements

Test email is successful

Kindly help.