Is it possible to send asynchronous emails with phpmailer?
Regular mail sending code snippet is as follows:
$mail->Send();
PHP waits
Update May 2016
As mentioned by user @Sinak Salek PHP does support multithreading. It is available using the pthreads extension.
Original
PHP does not support multithreading natively (which you need to do this beautifully). You can do it though by saving the emails in a database and then process them later using another script (e.g. using a cron job). In this way you don't have to wait for the underlying email framework.
Another thing, if phpmailer is slow it can be due to the underlying mail program (sendmail, postfix etc.) is setup incorrectly.
If you're on linux, you can put your php script in an exec command and launch it in background (put an & at the end of the command) and silent mode (2>&1 >/dev/null)