A grey ✓ usually just means the person hasn’t opened your email yet. But there are a few cases where an open can’t be detected.
Images are blocked
Read receipts rely on an invisible image. If the recipient’s email app blocks images — for example Outlook does by default for new senders, and some people turn images off — the open isn’t recorded until images are shown.
The email went to spam
Spam folders don’t load images. If your email landed there, opens aren’t detected until it’s moved to the inbox.
The email was read as plain text
Some apps and smartwatches show only the text of an email. No images, no open.
Tracking was off for that email
If there’s no tick at all, the email wasn’t tracked: tracking was switched off, a recipient had opted out, or you sent it from a phone or another app.
Only automatic opens so far
If the only activity was an automatic fetch (such as Apple Mail Privacy Protection) or a security scan, the tick stays grey on purpose. Open the email in your dashboard to see these events.
The open was yours
Opening your own email never counts. More on self-opens.