We did have trouble with the German provider freenet.de that just blocked all servers of the server park we have our servers in. They did unblock our IP addresses now, and freenet.de addresses seem to work now.
However, there's others that just block us for no reason. It seems to be a desperate attempt to reduce spam by only whitelisting the big ones, but it effectively results in blocking the larger part of the internet.
If you have trouble receiving our e-mails, turn to your eMail provider. Our servers are configured properly, they have never ever sent any spam, and they are not listed in dnsbl. We use all domain authentication schemes that are common, so there's really no reason to block icomp.de.
If your eMail provider is blocking us, he is most likely also blocking more legit sources. I strongly advise to switch to a provider that gives you access to all of the internet, not a selection that you have no control over.