Erm.....I don't quite get that. I have two GMX a/cs, and both work just fine in SeaMonkey.....after the hassle of setting-up all the config stuff manually. (T-Bird does spoil you in this respect - and I definitely prefer it! - but SeaMonkey certainly gets the job done with e-mail).
Are all the server settings correct? This does take some faffing-about with if, like me, you're more used to Thunderbird....
Mike.