The ideas listed here have already been implemented by GMX.

implemented
Vote
96
points
Show contact picture in email message Posted by Waxhaw to Mail-Features, 14 May 09 at 13:51
When receiving an email from a contact in my address book, I would like the contact picture I uploaded to show up in the email. [....]
implemented
Vote
2
points
Facebook contact import wizard is not working Posted by LEO to Miscellaneous, 22 Apr 10 at 17:20
Facebook contact import wizard is not working [....]
implemented
Vote
157
points
Plain Text feature Posted by HarrytheSmith to Mail-Features, 6 Apr 09 at 11:44
Hello again,

I am also missing a plain text feature. Not everybody needs HTML, or do they?... I don't. Hopefully you can add that.

With kind regards,

Adam Smith [....]
implemented
Vote
62
points
Attachments handling! Posted by andy to Mail-Features, 4 Feb 09 at 17:01
On occasions there are a lot of files in a single message. It can be burdensome to save these one by one. Therefore you should add a function with which the user can simply download all of the attachments with one click. [....]
implemented
Vote
42
points
Please reduce the size of the logo/banner on top of mail Posted by Joe163 to Design, 7 Aug 09 at 15:04
Very annoying that it needs to be that big. There is hardly any room for reading email.

And your response telling people to hit F11 is ridiculous, just fix it. [....]
implemented
Vote
77
points
cross browser support Posted by kai666 to Support, 3 May 09 at 21:34
Please make GMX compatible accross all browsers, like all the main email providers do. Not just for the 3 browsers favoured by GMX. If yahoo,aol, googlemail, hotmail and many smaller providers can have cross browser support, why can't GMX? [....]
implemented
Vote
16
points
No pop-up window/"Are you sure?" on Log out. Posted by ano to Design, 1 Dec 09 at 10:23
Popups are extremely annoying and do not make for a good "feature"... Whenever I log out (I have the 'Remember Me' option enabled) there comes out a pop-out window telling me that I am still singed in and have the option to log out or exit...

There should be no pop-ups. Or they should become optional just as you did with the sign in pop-up window. Alternatively, you can place 2 buttons: Logout and close so the user has the option to either log out permanently or just close GMX while viewing the inbox and not through a pop-up. [....]
implemented
Vote
89
points
Colour-Themes Posted by andy to Design, 31 Mar 09 at 10:19
GMX should add more colours to their themes eg. Orange, red, black etc. Those available are becoming dull. [....]
implemented
Vote
8
points
Allow sending BCC: only Posted by Tam to Mail-Features, 1 Sep 09 at 19:34
I prefer to send all my messages using BCC: only so that my friends email addresses are not harvested when a message gets forwarded. The GMX system requires an entry in the TO: field before the message will send. I find this aggravating and it will limit my use of GMX. Should be a simple programing change to eliminate the verification of the TO: line. [....]
implemented
Vote
452
points
CUT and PASTE Posted by Innovation to Settings, 22 May 09 at 20:57
I really think it is about time that GMX allowed us to copy and cut text by left clicking with the mouse. [....]