Is it our MTA which is incorrect by interpreting <CR>.<CR> as the same as
<CR><LF>.<CR><LF>
or is it the sending MTA/MUA which is incorrect by creating a message with
<CR>s in it and then not padding the '.'. (Surely if it really wants <CR>
characters rather than <CR><LF> line terminators, it should use
Quoted-Printable or Base64 encoding, not just put them in there as ASCII
0x0d characters)
your MTA is making a too-liberal application of the "be liberal in what you
accept" principle. unfortunately there used to be a lot of MTAs (mostly
misconfigured sendmails) that would send <LF>.<LF> (not sure about <CR>.<CR>)
rather than <CR><LF>.<CR><LF>, to terminate the message. many MTAs adapted
by accepting any of <CR> <LF> or <CR><LF> as line terminators.
the sending MUA probably shouldn't be putting bare <CR>s in the message either.
however, this is perfectly legal in RFC 822.
Keith