Why does Mail.app keep sent attachments open

If your mail account is IMAP or Exchange, you can simply rebuild your mailbox(es).

1. In Mail.app, ensure that the Mailbox sidebar is exposed and highlighted (click the mailbox you would like to rebuild)
2. On the menu bar, select Mailbox -> Rebuild to rebuild the highlighted mailbox.

Repeat, if necessary for any other mailboxes.

Based on the page http://lifehacker.com/5841097/how-to-delete-or-archive-attachments-in-apple-mail-and-free-up-disk-space I used the steps bellow.

** Method recommended only for those who feel comfortable using the Terminal **

• open the terminal app (Applications > Utilities > Terminal)

• change to the directory/folder with the emails typing in the terminal window

  $ cd ~/Library/Mail/V2
  

• list the folders you have

  $ ls
  

  or

  $ ls -1
  

• you will obtain something like:

  $ ls -1
  IMAP-info@furia.com.br@pop.furia.com.br
  IMAP-infofuria@imap.gmail.com
  MailData
  Mailboxes
  

• go to the folder of the account you want to clean, list the files and the result shows one or more mbox

  $ cd IMAP-info@furia.com.br@pop.furia.com.br
  $ ls -1
  INBOX.mbox
  

  or

  $ ls -1
  INBOX.mbox
  INBOX0.mbox
  INBOX1014.mbox
  

• if you have only one mbox, go to this folder, list it and you will see

  $ cd INBOX.mbox
  $ ls
  CE7AE9-C83C-410A-9406-4DEFB7034
  Archive.mbox
  Deleted Messages.mbox
  Drafts.mbox
  Info.plist
  Junk.mbox
  Sent Messages.mbox
  Trash.mbox
  

• change to the folder of the messages sent (note the backslash before the space) and list to see the files

  $ cd Sent\ Messages.mbox
  $ ls 
  CE7AE9-C83C-410A-9406-4DEFB7034
  Info.plist
  

• the first file is a folder and contains all the messages and attachments in a series of subfolders. Now you will use the powerful unix find command to find all the files to be deleted and delete them.

• the find command can recursively descends the directory tree to search files by name and by size (among other options), so if you know that the attachmentes to be deleted are PDF files with more than 200k you can use the first command to find and list the files. Then if it's ok, delete them with the second command. The second example will find and delete zip files with more than 1 MB.

  $ find . -iname "*.pdf" -size +200k
  ./77CE7AE9-C83C-410A-94/Data/Attachments/303/2/abc.pdf
  ./77CE7AE9-C83C-410A-94/Data/Attachments/305/2/xyz.pdf
  $ find . -iname "*.pdf" -size +200k -delete
  $ find . -iname "*.zip" -size +1M -delete
  

• and if you want to delete all attachments bellow the present folder, you can use one simple command. To confirm that you are at the correct folder verify first printing the working directory. Then, remove every Attachments folders bellow it.

  $ pwd
  /Users/user/Library/Mail/v2/IMAP-info@furia.com.br@pop.furia.com.br/INBOX.mbox/Sent Messages.mbox
  $ find . -iname Attachments -exec rm -R {} +
  

• repeat these operations on all mbox of the account