[Note: There's an updated post on this topic]
YSmtp, and I'm sure you know who owns them. Well, they are pretty good in filtering of emails, good in the sense that they always try to park all incoming emails in your bulk folder.
With a legit email and clean server, this is what happen when you try to send an email with attachment larger than 2MB into their network, and we wonder if this is how they can offer unlimited email space since huge (if you consider 10MB as HUGE) emails are guaranteed to be dropped.
And by the way, filing this as an issue to their so-called customer care is as good as sending letter to Atlantis.
 Connecting to 188.8.131.52
 Connection to 184.108.40.206 from 220.127.116.11:64570 succeeded
 RSP: 220 mta1048.mail.re4.yahoo.com ESMTP YSmtp service ready
 CMD: EHLO mail.elohkcalb.com
 RSP: 250-mta1048.mail.re4.yahoo.com
 RSP: 250-8BITMIME
 RSP: 250-SIZE 41943040
 RSP: 250 PIPELINING
 CMD: MAIL FROM:firstname.lastname@example.org SIZE=9797062
 RSP: 250 sender email@example.com ok
 CMD: RCPT TO:firstname.lastname@example.org
 RSP: 250 recipient email@example.com ok
 CMD: DATA
 RSP: 354 go ahead
 The smtp session has timed out.
Sometimes you would get this too:
 Connecting to 18.104.22.168
 Connection to 22.214.171.124 from 126.96.36.199:64585 succeeded
 RSP: 421 4.7.0 [GL01] Message from (188.8.131.52) temporarily deferred - 4.16.50. Please refer to http://postmaster.yahoo.com/errors/postmaster-21.html
 CMD: QUIT
1) The actual IP in log was not 184.108.40.206. We are using this IP to showcase the examples but feel free to do whatever you want with it. It is one of the IP that we blogged about in the bulletproof datacenter entry.
2) Email addresses in the examples are faked for safety reason. Again, you are free to do whatever you want to these addresses.
3) Whatever you do to the IP and email addresses listed in this blog entry has nothing to do with us.