Roland,
1) All connections to my server can be redirected through an attacking system by using ARP-poisoning techniques
Yes. And arp poisoning isn't your only worry. They could DoS your system, then steal your MAC address, too. ARP poison countermeasures won't help against that, only proper authentication of communicating parties will.
2) I can try to notice if something happens to the ARP-table but I can not prevent things from happening without having a secure switch at Puretec ;-)
And remember that switches aren't security enforcement devices and shouldn't be expected to be.
3) All unencrypted traffic can thus be read by the attacker
<nitpick> Oh, even encrypted traffic can be read by the attacker. He shouldn't be able to read the plaintext, though. </nitpick>
4) Even if I use a VPN to transfer all data between my internal network and my server at Puretec (thus being protected), Emails (i.e) will be exchanged with other systems on the internet and it is highly improbable that this traffic will be encrypted too.
That depends entirely on the other systems and the level of influence you have on them.
5) That way all emails will be readable to an attacker no matter what I do to protect them
Plaintext Internet traffic can be read while its underway and you have no assurance of the path that traffic will take. Neighbours of your server in the ISP's rack aren't the only ones capable of capturing 'your' traffic, though it is relatively easy for them. Don't forget, though, that what matters is not so much the possibility of this happening or its probability, but rather the risk you're taking. I.e. how much of a problem would it be if someone captured your traffic? This will dictate how much effort you should put into attempts to (perhaps partially) fix the problem. One easy method to solve the arp poison issue is to place the server in an environment under your own physical control. That won't help defend against hackers 0wn1ng the mail exchangers of the people you send email to, which may well be well-known ISP machines. Tobias