I am trying to send an email that is 5500KB. An error “Error while writing spool file” appears.
It seems that it is a server sided problem but send us your SMTP logs anyway. Go to menu Tools->Settings->Advanced->Logging and check “SMTP” under the problematic account. Then restart eM Client, try to simulate the issue and send us (with reference to this topic) the logs using the same logging settings window. Thank you.