I use a mock SMTP server (MockSmtp or fakeSMTP) on my Mac for debugging and testing mail delivery during development. These mock SMTP servers simply display the mails which are to be sent instead of delivering them.
Since upgrading to Yosemite, though, sending mails to this local SMTP server via JavaMail (tested on Glassfish V3 and JBoss AS 7) is painfully slow. A single mail takes about 20s to send. Sending a mail via netcat does not have any delay so the problem should not be caused by the SMTP server. I had no such problems with Mavericks. Mails sent on the production systems (Debian stable) are delivered quickly, too.
I am currently running JAVA JDK v1.7.0_71, the JavaMail implementation and the mail session is provided by JBoss AS 7.4 and mails are sent using Seam Mail (Seam v2.3, latest nightly build). Using seams own JavaMail implementation and mail session does not change anything. Disabling the firewall (Little Snitch) also does not help.
What has changed from Mavericks to Yosemite which could cause these issues?