As for mitigating these types of problems (besides the usual spam issues), there are various things you can do, from the simple to the extreme. I'm going to list a couple exampls. One simple, one extreme.
For the simple, you can contact the person who manages your mail server (ISP, webhost, etc.) and ask them what spam filters they have in place, and if they have any DoS/DDoS mitigation filters in place. If they don't, ask them what you can do to work together, exlaining the situation, and see what you can come up with.
For the extreme, you can setup a mail server at your place, have it retrieve your mail, and then force it through filters and so on. Then you can retrieve your mail off of that server. It is a mail router setup, and rather effective if done properly. I've done that at home, though it isn't quite finished yet. Like I said, extreme.
Typically mail bombings are a retaliation. For example, part of my job at work is to report spammers and mitigate spam problems (such as phishing, etc.). I reported one spammer (a well-known spammer, I just can't remember his name at the moment), got them kicked off of two different hosts. They obviously weren't too happy about it and mail bombed us. Was there a situation like that? Just a thought anyway.
----------------------------
"Security is like an onion" - Unknown