You can send expiration reminders to a lead/customer assigned to a proposal; email will be sent to the proposal email added while you created/edited the proposal.
To configure email expiration reminders navigate to Setup -> Settings -> Cron job-> Proposals.
Set Send expiration reminder to YES
Set expiration reminder before in days.
eq. BEFORE 4 DAYS – proposal open till date is XXX-09-22 and expiration reminder will be sent on XXX-09-18.
Expiration reminders will be sent only on proposals with status OPEN and SENT.
You will need cron job configured for this feature.