During sprint planning, the increment must be deployed at the customer's site at the end of the sprint. The product owner received an email from the account manager saying the customer may not be ready to install the new version. What should the project manager do?

The project manager should contact the customer's project manager because this is the most appropriate and formal channel for discussing potential changes to deployment plans and understanding the customer's readiness. The customer's project manager is responsible for their organization's side of the project and can provide accurate information and decisions. Asking the sales team is indirect and may not yield the necessary technical or logistical details. Asking the product owner to inform the customer shifts the responsibility and might not be as effective as a direct PM-to-PM conversation. Contacting the customer's technical lead might be appropriate later for technical details, but the initial communication about a potential delay or change in deployment strategy should go through the project manager.