[layer8]

I generally agree, but the two consecutive dots (or leading/trailing dots) are an example that would very likely be a typo and that you wouldn’t particularly want to send. Similar for unbalanced quotes, angle brackets, and other grammar elements.

[dumbo-octopus]

I wonder whether simply (regex) replacing a sequence of .'s with a single one as part of a post-processing step would be effective.

[layer8]

That would be bad form, IMO. The user may have typed [e-mail] by mistake instead of [e-mail], and now you’ll be sending their email to [e-mail]. Similar for leading or trailing dots. You can’t just decide what a user probably meant, when they type in something invalid.