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
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
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
5
points
Messages saved in Dafts marked as Unread Posted by eusty to Mail-Features, 18 Aug 09 at 17:01
When a message is saved to Drafts it's marked as read.

It would be better if it's automatically marked as unread when saved to the drafts. This would then appear on the folder view as Drafts (1) [....]
implemented
Vote
108
points
"Are you sure" message Posted by Waxhaw to Mail-Features, 14 May 09 at 13:56
When closing an email that has not been sent, I would like a pop-up message asking "are you sure". This would save me from losing the message if I accidentally try to close it. It should also have an option to turn this message off. [....]
implemented
Vote
9
points
Spam-Folder Posted by andy to Mail-Features, 30 Mar 09 at 14:22
If the spam protection is disabled, the folder on the left hand side should not be visible. [....]
implemented
Vote
45
points
subject line: stop adding "Re:" on reply Posted by firelovesoul to Mail-Features, 8 Apr 09 at 05:47
Place a script in that looks for "Re:" at the beginning of a subject line => if yes - DO NOT add another "Re:" => if no add "Re:"

Why? Subjects that read "Re: Re: Re: Re: Re: Re: Re: re: Re: An important subject" can get quite annoying. [....]
implemented
Vote
183
points
No pop-up window when signing in Posted by JohntheBaker to Mail-Features, 31 Mar 09 at 16:19
Dear GMX,

if you could add a function with which users could either login with the pop up window or directly into the browser, it would certainly be greatly appreciated. Most likely it would ensure a greater convenience for mobile device users!

Thank you very much! [....]
implemented
Vote
167
points
The GMX Homepage should remember my password Posted by JohntheBaker to Mail-Features, 31 Mar 09 at 11:17
Dear GMX,

a great addition to your service would be a "remember me" function. With it I could check my e-mails several times each day without having to re-enter the information every time.

Thank you very much. [....]