Does it look like all or most of your list bounced?
The first thing to check for is that there were no formatting errors on import. Sometimes .csv files need some extra love before they are imported. In some cases, you'll see the file try to cram a bunch of information into the email field instead of separating the columns out properly. If you go to the Lists page in your account and click the name of the list to view its subscribers, you'll see really quickly if the email addresses are formatted correctly. This can happen, for instance, when an open quotation mark (") occurs in a spreadsheet. So if you have "email@example.com and then no closing quotation mark ("), the file may try to cram everything on that line into the first column. A search for the phrase "remove quotation mark from excel" online may yield some helpful results.
Did the campaign use a yahoo.com or aol.com email address for the From email?
In an effort to stop fraudulent emails, AOL and Yahoo no longer permit third-party senders like MailChimp to send on behalf of aol.com or yahoo.com email addresses. Because of this policy change, campaigns using an aol.com or yahoo.com domain for their From email address will fail DMARC authentication, causing emails sent to receiving servers that check for DMARC to bounce.
While using another free email service like Gmail or Hotmail could be a potential short-term solution, other free services are likely to adopt similar policies in the future. We strongly recommend an email address from a private domain like the one you use for work or your website.
If you have had this list for a while and are starting to see high bounces, it's possible the list is starting to go stale.
If you feel the list could be stale, maybe it's time to reconfirm your inactive subscribers.
If this is the first time you have used this list and are seeing high bounces, it's possible the list was combined from multiple sources or contained old addresses.
We recommend reading this article that'll help you to determine whether your list is all right to continue using: Is my list okay to use in MailChimp?