You can send expiration reminders to the lead/customer that this proposal is assigned, email will be sent to the proposal email added while you created/edited the proposal.

proposal-email

To configure email expiration reminders navigate to Setup->Settings->CRON job

  1. Set Send expiration reminder to YES
  2. 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

NOTES:

  1. You will need cron job configured for this feature.
  2. Expiration reminders will be sent only on proposals with status OPEN and SENT