JavaMail Programming Best or Better Practice

I have about 1000 user accounts in my application in company. The users besides their intranet emails have also internet emails(via POP3, SMTP, IMAP). There is a Java application, which I call worker, checks those users internet emails. The problem is, since the user accounts size is big for checking external emails, it takes too much time to complete the checking of开发者_StackOverflow社区 all the accounts. Is there any best or better practices or patterns for solving such type of problem.

This is not an answer. You could look how Thunderbird does things, delve into its code. I know DNS lookup does take its (small amount of) time; a simple sort on the domain might help. Or not if the addressed mail server then receives a batch of mails. You could install local mail servers and thus take load of your java app.