JavaMail编程最佳或更好的实践

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 all the accounts. Is there any best or better practices or patterns for solving such type of problem.I have about 1000 user accounts in my applicati