Certain fields only visible to admin?

Make the fields E-Mail login and password visible only to the site administrator.

That might be possible, by adding a permissions settings. Then you (or anyone else using this module) could use that permission to hide the information, or even delegate that task to an admin role.

This way I can enter the data for the users without having to muck about with AES (i.e. retrieving the user PW) AND what is even more important the E-Mail will function also if the user changes his site password.

I know that there is the option to let the user enter his own login and password - but since he can't create E-Mail accounts at my provider that will not do him any good.

I use the Courier MTA module for that. I realize that may not work for your situation, but there is at least one other module (and maybe more) that might help there, i.e: Mailfix for Postfix (but maybe not, as well).

Two things I've thought of doing in the past were:

1. Creating a module that could be used by any e-mail based module as the generic method of handling access to users' e-mail account (maybe the AES module could be that module -- I haven't investigated it enough to know).
2. Creating a generic e-mail server access module that could have plugins for various protocols (SMTP, POP3 etc.), and/or server software (sendmail, qmail, Courier, etc.).

Having both of those available would, I believe, take care of everything you were looking for above. I haven't had the time to maintain the stuff I already have, much less create something new (at least the second module would be quite an extensive and time consuming module to create correctly) to maintain, so they haven't gone anywhere.

Just a minot nuisance. When I access the webmail and I see my inbox a click on one of the E-Mails should open up the E-Mail. What is does now is to open up the header a litte more and then offer the option to "View this message as a web page".

Actually, it already opens an E-Mail with one click. What it does is open the plain text portion, first. You are only seeing a small portion of the e-mail because the e-mail is not completely formed and only has an HTML part. If it were completely formed and had both parts, you would see everything in the first opening -- just not the HTML formatting. I did that on purpose to reduce the likelihood of individuals accidentally executing something that could harm their systems. I suppose I could provide an option for the administrator to select which to show first.

I'm not actually likely to implement any of the features any time soon, due to my already existing back log of work, but they may prove to be simple enough features that I might implement one or more of them as I'm doing other things.