Sent email occasionally not syncing with iCloud

Hello,

I sent an email from the Mail app on my iPhone. I could see the email message in the Sent folder on my iPhone, but I didn't see it in iCloud Mail (in the browser). I did receive a response to my email, so it did go out.

A similar thing happened again.

I sent another email. I could see the email in the Sent folder on my iPhone, but I didn't see it in the iCloud Mail (browser). I tried to figure out why it happened. I deleted the Mail app. I saw a message telling me that local data would be removed, but iCloud data would not be. This makes sense and I didn't think a lot about it.

When I installed the app, I noticed the email message was no longer in the Sent folder on my iPhone.

I think I know what happened. The message I sent was only stored locally, and when I deleted the app, the message was also deleted.

Likely, the message was not synced with iCloud.

Does anyone know what causes a particular message not to sync with iCloud?