Hey there
Not sure if this has been discussed before, but a solution I have used to combat the sending to hotmail problems is as follows:
After speaking to MSN/Hotmail themselves to ask what possible problems could be, we have gone through several problems...
1) SPF (make sure there is a SPF record in your DNS tables for the domain you are sending mail from)
2) Charset
3) Body text length
Originally they presumed it what going to be to do with 1) SPF after testing from some of my domains with and without SPF - I found this didn't make too much difference (although I recommend always having SPF records to help with global efforts of SPAM)
Secondly, we tested Charset - finding out that RoundCube defaults to UTF-8 was quite a problem, not necessarily ours (users and makers of RoundCube) but with Hotmail, it seems messages sent to them using UTF-8 could be blocked...I tested using ISO-8859-1 (Latin-1) and things got better...
I therefore updated the file /program/include/main.inc - the following section:
// language has special charset configured
if (isset($rcube_charsets[$lang]))
$charset = $rcube_charsets[$lang];
else
$charset = 'UTF-8'; ----> I changed this to ISO-8859-1 from UTF-8
So its now $charset = 'ISO-8859-1';
I understand that some language settings will change this default charset setting but having it default to ISO seemed to work for using UK/GB/US English. This seemed to correct most of the problems.
The only other things we tried with Hotmail were Subject, and Body length - if an email is too short or doesn't have a subject it also seems to cause issues with Hotmail.
I'm not sure if this is a total confirmed fix to this problem, but its working for me and my webmail users.
Thanks
Dominic