[exim] smtp timeouts / what to look for

Dear all,

I have been getting smtp timeouts from certain domains (mostly Yahoo)
and I at first I thought I had been blocked as mail to 90% of other
domains is flowing well (gmail, aol, you name it). It took a lot of
time to communicate with Yahoo but they have eventually replied
stating that they are not blocking us.

But even a simple "telnet a.mx.mail.yahoo.com 25" times out. We have
never had that issue before. If they are not blocking us, what else
could I check to be the cause of networking problems? If it matters, I
can telnet yahoo.co.uk and yahoo.co.cn (or yahoo.cn I don't remember
now) but they seem to be using different MX-s.

I am on a FreeBSD 6.3-RELEASE-p1. But this is hardly OS-related issue.
We're behind a Dlink DFL 700. Telnet to yahoo works on no machine,
whether it is Windows of FreeBSD.

Our DSL provider claims the line is OK and there are no issues. The
DFL utility has been with us for 2 years or so and seems to be working
OK.

exim -bV
Exim version 4.69 #0 (FreeBSD 6.2) built 03-Jan-2008 07:47:57
Copyright (c) University of Cambridge 2006
Berkeley DB: Sleepycat Software: Berkeley DB 4.2.52: (December 3, 2003)
Support for: crypteq iconv() IPv6 use_setclassresources PAM Perl
Expand_dlfunc OpenSSL Content_Scanning Old_Demime
Experimental_DomainKeys
Lookups: lsearch wildlsearch nwildlsearch iplsearch cdb dbm dbmnz
dnsdb dsearch mysql nis nis0 passwd
Authenticators: cram_md5 dovecot plaintext spa
Routers: accept dnslookup ipliteral manualroute queryprogram redirect
Transports: appendfile/maildir/mailstore/mbx autoreply lmtp pipe smtp
Fixed never_users: 0
Size of off_t: 8
Configuration file is /usr/local/etc/exim/configure

Many, many thanks for hints how to troubleshoot the problem.

--
Zbigniew Szalbot