codeigniter email vs hotmail & gmail

[eluser]druid100[/eluser]
i ran into problems sending email with codeigniters native email lib to hotmail/googlemail. the emails i send just do not show up in hotmail/gmail ... neither in the inbox folder nor in the spam folder and they do not return to the sender address.

if i send emails from thunderbird/outlook to homail/gmail they arrive properly, so i'm sure the problem is not related to my outgoing mail server.

i started fiddling around with adding more headers ("Sender:" ...) and chaning the sender agent from codeigniter to a valid thunderbird tag etc. but without success.

do you guys out there ran into a similar problem?

any solutions?

any thoughts?