Are you using the built-in (it's typically on by default, at least it has been through most recent iterations of iOS, I'm assuming that it probably still is today) Photos app integration with iCloud? The one where it uses your iCloud storage space to back up all your photos automatically, with the choice of whether or not you want to use a feature called "My Photo Stream"...if you use this, then by default most of the time your photos should actually already be a compressed version that is of less quality versus the original image file. If you select the photo(s) from within the "Photos" app and use the share extension (that little square with the arrow pointing upwards out of the top, on the bottom bar of the screen) to copy/paste them into the body of your outbound e-Mails, or drag-and-drop them, it should send out the ones that your phone is automatically optimizing and not the full-sized ones.
Alternatively, you can try this app (it's totally free and there's no annoying premium things to pay for unlocking features or removing a watermark or anything like that, and no subscription nonsense), it's called Kompressor. I have used it for a while with good results, it's simple but it works. I choose to use it because it seems that by default when selecting the images through the 'attach items' option within Mail, it sends a request to the iCloud servers to download the full-sized version of the photo to send it to your recipient. Here's the link to the app: https://apps.apple.com/us/app/kompressor-compress-images/id1011979611
Note: you can tell whether or not the image(s) you have in your photo albums are in fact currently existing in their original, full-size versions or if they are in an optimized smaller file by looking for the presence of the little cloud icon in the bottom corner of the image when going through them in the album gallery.