I would start with Power Automate .. but
One of the best features of the Power Platform Portal is that it is integrated tightly with a large list of functions and features (1000s and choices 1000s). Thousands upon thousands, but the disadvantage is that you need to understand what the options are and what works for your users and needs.
When it comes to e-mail you can leverage the power of "Server Side Sync" which is the integration between Power Portal and the Full Microsoft Exchange Server environment. Wow, Configuration (such as mailboxes for the system accounts to send system mail) and/or licensed user mailboxes so they can send mail without using outlook.
Another option is to look into using Power Automate to send mails using the integration to the Office 365 suite. This is really more about the shift from a world where we have office 365 and different business applications to the world of Microsoft 365.
So Option 1: Read up on configuring the Server Side Sync.
a) Settings : Configure Mail area (there are a number of configurations needed, (no code) but helps understand ALL the pieces. I would give you exact menus, but they are moving things around so better to lean on DOCS (which is awesome) https://docs.microsoft.com/en-us/power-platform/admin/server-side-synchronization
Option 2: Power Automate (alias Flow) which you would need to trigger from your Power Portal. This might be easier configuration and a fun way to learn (no code, low code) flow. https://docs.microsoft.com/en-us/power-automate/