Issue 1: Using [at] as a munge.
The whole [at] thing and even using images are no real challenge to some of these tools. There are plenty of munge tools out there, only a few are really worth the effort. You can search Google for e-mail munge for a list.
Please(!) munge your e-mail addresses. Do not rely solely on anti-spam software. Get them at the source. Once they have your e-mail address, there is no stopping them.
Issue 2: Is [at] harmful for search.
I warn you that there are a BUNCH of wing-nuts out there spouting all kinds of SEO stuff (not the word I want to use) that is not only wrong but ridiculous. Where-ever you heard this, please avoid this so-called resource at all costs. There is no credibility that can be attributed based upon what they told you. The short answer is no. It will not effect search.