I am agree with you!
Before I sent only one email with one transport, but it is very slow to create and init one transport. Since that I determine to send more emails with one transport. It is more quick than before. And than I meet the problem you see in the topic. To resolve it I start to use arithmometer. When one transport sent 20 emails ,a new transport will be created and continue the work. Now it can work correctly and quickly!
But I think I will encounter another problem: Now the email server can allow one transport to send 25 emails, but maybe someday the number will change to 20 or 15,and than that problem will appeare again! So I need a method to catch the exception resolve the problem! Do you have any good idea?