I block mail with a trailing dot in the RCPT_TO field.
I use these rules, and they do well.
SLocal_check_rcpt
R<$+ [at] mydomain.ru.> $#error $: "553 Trailing dot in
the recipient address."

But does it break any RFC ?

The reason of blocking is in the filter spamooborona whitelist.
Spamooborona works only for mailboxes listed in the whitelist.

So if mailbox user [at] mydomain.ru is in the whitelist,
and mail is with user [at] mydomain.ru. in the RCPT_TO,
spam will reach the user.

Received: from killersoft.com ([122.43.86.163])
by mail.mydoman.ru (8.13.6/8.13.6) with SMTP id l3FAToYq001563
for <postmaster [at] mydomain.ru.>; Sun, 15 Apr 2007 16:29:53 +0600
....
....
X-Spam-Flag: SKIP
X-Spam-Yversion: Spamooborona 1.5.2

In article <1187876789.404392.187780 [at] i13g2000prf.googlegroups.com>
Sciurus <sciurus [at] mail.ru> writes:
>I block mail with a trailing dot in the RCPT_TO field.
>I use these rules, and they do well.
>SLocal_check_rcpt
>R<$+ [at] mydomain.ru.> $#error $: "553 Trailing dot in
>the recipient address."
>
>But does it break any RFC ?

Not AFAIK - that is certainly not valid syntax for an email address.
Just be careful with the fact that sendmail.cf internally uses a
trailing dot to indicate "canonical domain part" - testing the RCPT
argument directly as you do should be fine, but if some standard ruleset
processing has been done you're likely to have a trailing dot on all
valid addresses (it is removed on "output" of course).

--Per Hedeland
per [at] hedeland.org