Why write 'at' and 'dot' in email rather than '@' and '.'?
Typically people do this to avoid having their email addresses found by spambots that search for a regex that matches email address formatting. By using _AT_ and _DOT_, the symbols that the spambot is looking for will not show up on the page, and therefore your email address will not be found.
It's generally good practice if you want to avoid an inbox full of spam. and you give out your email address on public pages. However, you can avoid the issue altogether by not posting your email address on a public forum. If you want to give it to someone, send it by a private message or email or something.
This is to obfuscate the Mailadresses from Mail-harvesters.
This is one of the many anti-harvesting techniques used against automated spambots. This one is used massively, so there are lot of spambots used to it and it does not work 100%. You can see some statistics and comparison of various anti-harvesting techniques in this great article
<a href="mailto:[email protected]">[email protected]<span style="display:none;">ignoreme-</span>example.com