[Solved] New mail messages are not sent and remain in the Drafts folder

Hello everyone, I’ll describe the issue and the checks that have been carried out in detail:

• Many users in the company where I work—actually almost all of them—are reporting that some emails are not being sent. This happens when sending messages from the web app, from a client (Thunderbird), and also from the mobile app. The messages remain in the Drafts folder.
• After several tests, incorrect user behavior has been ruled out (for example, not waiting for the green progress bar on the right side of the screen to finish).
• Connectivity tests were also performed on the workstations most affected by this issue, and nothing abnormal was detected (for example, using ping -t showed no lost TCP packets).
• Important: we found that when an email remains in Drafts instead of being sent, the popup asking whether to send the message without the attachment does not appear, and the green progress bar indicating that the message is being sent does not appear either.

Thanx for help