To keep a copy of the message you send, activate the checkbox next to “Save”. The destination mailbox will default to the mailbox selected for this purpose under;
“Options-> Personal Information-> Identities“.
Saving copies of important e-mails you send could be useful for record keeping of what was discussed. This feature can be activated by default by going to
“Options-> Personal Information-> Edit your identities“.
In your options you can also setup maintenance tasks that empty or rename these sent mail mailboxes periodically.
You may not want to save every e-mail since you could have problems with your disk storage quota being used up quickly, especially when you send large attachments. A mailbox with full storage quota will not be able to receive new incoming e-mail.