Why am I seeing the error: "sbm:Too many query rows: 50001"

You are seeing this error due to the cap on the number of updates that Salesforce will allow on a DML.  The max is 50K.

You can reach out to Salesforce to see if they can make an exception for your org to increase your query rows governor limit to 100,000 rows.

Here is language that you can use to make that request:

Our organization is using a Salesforce AppExchange app that we’ve found we’re running into a governor limit that we’re hoping you can increase for us.  We are reaching a "Too many query rows: 50001” governor limit error when utilizing the app, and at this time, the app provider (Soapbox Mailer) has shared that it’s required that we leverage the query in a non-batch state in order for the app to function for our needs.  Can you increase our governor limit for SOQL query rows to be 100,000, even if it’s just in the context of this app namespace (sbm)?

Alternatively, you may want to consider either breaking things out into several matching campaigns, or adding and sending to smaller groups of Contacts at a time in a single campaign.