Exchange SMTP relay backup

Currently running a hybrid environment where user accounts are created in Active Directory (AD) and synchronized with Azure AD (AAD). All of our mailboxes are hosted in Exchange Online, but we still rely on an on-prem Exchange 2019 server for SMTP relay to handle notifications for internal apps. The problem is that this has become a single point of failure.

To address this, we’re planning to add a secondary SMTP relay server for redundancy and high availability. The plan is to set up the backup environment without affecting the existing one for testing, before fully implementing.

We’re considering using IIS for SMTP for the backup relay. Any advice or recommendations on using IIS for this purpose, or would it be better to set up another Exchange server for the backup? Appreciate any insights!