If you're uninstalling Soapbox Mailer and have recently (within the past few days) deleted and emptied out of the Recycling Bin a Campaign that had a Soapbox Mailer record type, you might receive an error as follows:
"Unable to uninstall package" with additional error messages that say, "The installed component is referenced by a locally created component" followed by the name of the Campaign record.
The reason is that you have a record in Salesforce that is essentially in a quasi-permanent deletion state. Even after emptying your Recycling Bin, your data records still exist briefly in Salesforce's cloud. At this point, since you can't undelete the record to change the record type on the campaign record, you'll need to open a support case with Salesforce to ask them to permanently delete any recent records emptied from the Recycling Bin.
Once Salesforce completes that (typically within 48 hours), you'll be able to uninstall Soapbox Mailer completely.