Apple mail fails to send mail GL.iNet GL-X3000 with cellular

Technical Support for Routers 4G LTE, 5G NR, USB modems
1

Apple mail iCloud account with default email server settings using the Apple Mail client on a MacBook.

After router firmware upgrade and reboot, sent mail from an iCloud account is appearing in the sent mail folder but is NOT being sent. Gmail account in the same Apple Mail client with default gmail server settings is working correctly. RESOLVED, at least temporarily as described below.

GL-X3000 kernel version 5.4.211 cellular only connection using a free.fr 5G SIM in France.

It is possible that the Apple SMTP servers are dropping the emails as possible spam.

Resolved:
After trying every possibility of forcing a reconfiguration of the iCloud SMTP and IMAP servers, I tried a last resort power off of the router (not a reboot) and waited 3 minutes before powering back on. This has reset the router connection to the cellular network and has fixed the mail problem for now.

2

iCloud Mail should be normal TCP/UDP traffic.
In addition, the GL router does not intercept any traffic.

It may be the MTU value (related to the operator). If the problem occurs again, can try setting the MTU: 1500, 1420, 1372, 1280, etc.