For about a week I have not been able to open attachments or save them, and some I have saved appear to be empty. I have tried to rebuild the mailbox but this is not responsive (OS Sierra 10.12.3) Has anyone experienced the same problem?
MacBook Air, macOS Sierra (10.12.3)
Try re-indexing the mailboxes. This can take awhile if you have a lot of mail.
Reindex messages For El Capitan, try looking in V3. Sierra use V4
Reindex messages (2) See post by Linc Davis
You may have to go to the "next" step which would be to repair the mailbox in question.
Note: With macOS Sierra the Apple Mail folders ares now located in: ~/Library/Mail/V4/MailData/. The below articles still reference earlier builds located in either V2 (OS X 10.7 - 10.9) or V3 (OS X 10.10 - 10.11).
Ref:
I cannot rebuild my mailbox
