Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Manage E-Mail Attachments with Mac Mail
Messages with attachments can be rather large and take up a lot of mailbox space. Of course you can remove these e-mails, but what if you still need some information from them? One way is to copy the message texts to a separate document. However, Mac Mail offers a more convenient way. You can receive all e-mails with attached files to a separate folder and store them in it. Moreover, you can remove attachments from all of these messages at once. Follow these simple steps:
1. In Mac Mail, click Mailbox, New Smart Mailbox.
2. Enter the mailbox name, such as “Messages with Attachments”.
3. Don’t change the default “Contains [messages] that match [all] of the following conditions” condition.
4. From the drop-down under this filter, select “Contains Attachments” instead of “From”.
5. Click OK.
6. Mac Mail will create a new smart mailbox named “Messages with Attachments”. Only e-mails that contain attached files will go there. This will make it easier for you to manage the attachments and decide which messages to delete and which to leave.
7. To remove attachments from multiple e-mails, select all of them and click Message, Remove attachments.