Why email keeps rejecting your video
Gmail caps outgoing attachments at 25 MB total, and because email attachments are base64 encoded, a file that's comfortably under that limit on disk can still be rejected: encoding inflates the actual bytes sent by roughly a third. Other providers set their own limits, so check the specific cap for wherever you're sending before assuming Gmail's number applies.
Target the limit exactly
Rather than guessing at quality settings, QuickCompress's target-size mode lets you type the number you actually need, say 20 MB to leave headroom under Gmail's 25 MB cap, and it works out the bitrate to hit it. Everything runs locally on your Mac using Apple Silicon hardware encoding, and your original file is left untouched.
If the recipient needs full quality
For a video worth sending at full resolution, a cloud link is usually a better fit than an attachment either way. See compressing for Google Drive and sharing links, where a smaller file still helps: it uploads faster and uses less of your storage quota.
Sending to a group chat instead of email? The same target-size approach applies. See compress video for Discord and compress video for WhatsApp.