I totally agree, that mIRC should have a built-in spell checker.

I understand that it takes time to implement spell checking in mIRC, but I do hope to see such feature one day in mIRC.

And for now, until mIRC gets a built-in spell checker, people can use:

1. SpellCheck v1.1 by asus69:

http://www.mircscripts.org/comments.php?cid=2339

2. mIRC Spelling & Grammar Check script by SReject:

http://hawkee.com/snippet/8748
http://forum.swiftirc.net/viewtopic.php?f=34&t=24945