I have my e-mail forwarding set up in FSF's mail server to my account in Yahoo!'s mail server.
I tune into several mailing lists, and I noticed that frequently the e-mails came out-of-order if not delivered at all.
For example, from gcc-digest mailing list, I missed issue 5694 for almost a week:
gcc Digest 2 Sep 2008 15:39:07 -0000 Issue 5694 ----------------------------------------------- From ...<snip>...@gcc.gnu.org Tue Sep 2 08:39:07 2008 Received: from 188.8.131.52 (EHLO mail.fsf.org) (184.108.40.206) by mta189.mail.re3.yahoo.com with SMTP; Sat, 06 Sep 2008 06:51:32 -0700 Received: from sourceware.org ([220.127.116.11]:50393) by mail.fsf.org with smtp (Exim 4.63) (envelope-from ) id 1KaXyi-0001OL-Nf for ...<snip>...; Tue, 02 Sep 2008 11:39:37 -0400 Received: (qmail 9803 invoked by alias); 2 Sep 2008 15:39:07 -0000
As can be seen from the message header of issue 5694, mail.fsf.org had received the e-mail on Sep 2, but it kept the e-mail for almost a week until it forwarded the e-mail to my inbox on Sep 6.
This out-of-order delivery was very annoying because it disturbed my work. For example, I sent e-mails to my friends promoting Ogg is on the Rise! through a mailing list. My friend, A, replied me at time X. Later, another friend of mine, B, replied me at time (X + 1 hour) saying that B added further questions referring to those in the e-mail that A had sent before. However, A's e-mail was delivered 11 hours after B's e-mail. Therefore, I could not answer B's e-mail until after I received A's e-mail. Besides, I could also miss vital information along the way if the e-mails were just lost.
I suspected that there was something wrong with FSF's mail server and reported this problem to membership at fsf dot org.
It turned out that the right place to ask help for this kind of problem is sysadmin at gnu dot org instead of membership at fsf dot org.
And, the explanation of this problem from the sysadmin was that Yahoo! were randomly rejecting my e-mails that were forwarded by FSF's mail server. For example, one excerpt from the log of FSF's mail server:
2008-09-02 11:39:37 1KaXyi-0001OL-Nf <= gcc-digest-return-148887-...<snip>...@gcc.gnu.org H=sourceware.org [18.104.22.168]:50393 I=[22.214.171.124]:25 P=smtp S=71487 email@example.com T="gcc Digest 2 Sep 2008 15:39:07 -0000 Issue 5694" from for ...<snip>... 2008-09-02 11:39:37 cwd=/var/spool/exim4 3 args: /usr/sbin/exim4 -Mc 1KaXyi-0001OL-Nf 2008-09-02 11:39:41 1KaXyi-0001OL-Nf SMTP error from remote mail server after initial connection: host f.mx.mail.yahoo.com [126.96.36.199]: 421 Message from (188.8.131.52) temporarily deferred - 4.16.50. Please refer to http://help.yahoo.com/help/us/mail/defer/defer-06.html 2008-09-02 11:39:43 1KaXyi-0001OL-Nf SMTP error from remote mail server after end of data: host f.mx.mail.yahoo.com [184.108.40.206]: 451 Message temporarily deferred -  2008-09-02 11:39:44 1KaXyi-0001OL-Nf SMTP error from remote mail server after end of data: host e.mx.mail.yahoo.com [220.127.116.11]: 421 Message temporarily deferred - 4.16.51. Please refer to http://help.yahoo.com/help/us/mail/defer/defer-06.html 2008-09-02 11:41:19 1KaXyi-0001OL-Nf SMTP error from remote mail server after end of data: host b.mx.mail.yahoo.com [18.104.22.168]: 451 Message temporarily deferred -  2008-09-02 11:41:19 1KaXyi-0001OL-Nf SMTP error from remote mail server after end of data: host d.mx.mail.yahoo.com [22.214.171.124]: 451 Message temporarily deferred -  2008-09-02 11:41:19 1KaXyi-0001OL-Nf == ...<snip>...@yahoo.com <...<snip>...> R=dnslookup T=remote_smtp defer (-46): SMTP error from remote mail server after end of data: host d.mx.mail.yahoo.com [126.96.36.199]: 451 Message temporarily deferred - 
The sysadmin said that this might be the result of an anti-spam mechanism. And, although the sysadmin had already contacted Yahoo! at http://help.yahoo.com/l/us/yahoo/mail/postmaster/defer.html as specified on http://help.yahoo.com/help/us/mail/defer/defer-06.html, the sysadmin doubted that Yahoo! would respond in the near future. In other words, Yahoo!'s spam filter was smarty-pants, and I had to hack around it.
Changing to Google Mail was not an option because I did not like its UI: too sophisticated. And, strikingly, Google Mail boasted about its spam protection. What a big `NO' sign.
Looking around, I found GMX. This too boasted about its 7-layers of spam protections. But, I had never tried its UI before, and who knows, its spam protection might not be as dumb as Yahoo!'s. So, I gave it a try, and... gosh... its user interface was even more bloated than Google Mail. But, what a surprise: its 7-layers of spam protections could be turned off! So, this was a big `YES' sign. But, still I had a problem with its bloated UI.
After a day, I realized that GMX provided POP and SMTP supports while Yahoo! could be configured to fetch e-mails from other accounts through POP. So, I made my Yahoo! account fetch my e-mails from my GMX account. This way I have by-passed Yahoo!'s dumb spam filter and nicely solved this out-of-order mail delivery problem.
Oh, if you ask me why I use Yahoo!, it is because it has a simple UI, unlimited storage, and give you a feature to send e-mails through your Yahoo! account as if they were sent through your other account. This feature is important to me since it gives an illusion that my mail in FSF is a true account. Specifically, the e-mails will have its `From' field indicating my mail address in FSF instead of having my Yahoo! mail address the `From' field and my FSF mail address in the `Reply-To' field.
In short, I know I can trust FSF's mail server to perform its job properly. For this moment, the best arrangement for my e-mail delivery system is to have FSF's mail server forward my e-mails to my GMX account so that I can take them to my Yahoo! account through POP. For the sending system, I compose e-mails and have them send with Yahoo!'s feature to mask the address of my Yahoo! mail address while exposing my FSF mail address for my free software campaign.