I'm just wondering if you have a copy of all your emails offline?
I'm not a fan of keeping data on 3rd party services, say an ISP closed down my server due to X or Y i've got a backup of ALL my websites, data and configuration. I keep almost everything on my server and don't trust 3rd parties to keep my data.
I've got an aweber account but don't trust them to keep my big lists and always take a copy of all the emails (even if i have to get them manually / scrape the data)
A properly configured server / dns (SPF records etc.) shouldn't have issues handling the number of emails you have. You should then check the email logs for bounced messages and remove those from your list (or if there is another problem, resolve that issue).