To understand this, we must understand how crawlers find the email. While steering away from the technicals, the basic idea is this (today's algorithms are, of course, smarter than that):
@ in the page.
- Is there a dot within 255 characters after the
- Grab what's behind the
@ until you reach a space or the beginning of the line.
- Grab the
. and what's behind it until you reach the
- Grab what's after the
. until your reach the end of the line or a space.
Now, an easy countermeasure would be to replace the
at and the
dot. The most intuitive counter-countermeasure would be to teach the crawler that
at is actually
@. Well, it's not that simple. Take the following text:
We climbed into the attic and found a dotted piece of wood. Please email us: adnan at gmail dot com.
Now let's run our new crawler on it. First it will find the
attic, then it will find the
dotted. The resulting email would be
firstname.lastname@example.org, then it will find the second email
email@example.com. Then spammers started teaching crawlers about finding certain domains, ignoring spaces, taking spaces into account, considering certain domain names, etc.
Having that said, the most basic techniques usually give good enough results (apparently, in some place in the world, that link is NSFW. Personally, I disagree), and the more obfuscate, the better results you get.
So, to directly answer your question: Is using 'dot' and 'at' in email addresses in public text still useful? Yes, I think so, at least to some degree. But this solution has been around long enough for us to assume that most crawlers have already found a way around it.
My advice? Either use some fancy advanced munger, or simply use images.