It’s not your laptop that’s complaining. Exactly who is complaining depends on how you’ve got your email accounts set up, and which account you’re sending it to.
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A bounce happens when a mail server has to reject a message it’s receiving for some reason.
Since it’s complaining about an inbox being full, we can assume that the mail server that’s complaining is the your ISP’s mail server that hosts the email account you’re attempting to send to – the one your laptop would download mail from (i.e. the POP3 server and account settings as configured in the laptop). That might be the same server that you’re using to send from (the SMTP server and account settings you’re using on your desktop), but it might not be.
So why is it complaining about an inbox when the inbox is on your laptop? Because you have an inbox on your ISP’s mail server. The mail server for your account has to have a place to hold your email until you come along with Outlook Express and download it.
Say you don’t check email for a few days, but email is still arriving for you. That email is kept, in an inbox, on your ISP’s mail server. When you download your email, you are in effect moving it from the inbox on your ISP’s server to the inbox in your mail program.
Many ISP’s place a “quota”, or a maximum amount of email that you may have in your server’s inbox at any one time. Normally simply downloading email into your mail client periodically is enough to keep that server’s inbox empty as mail is moved from it to the inbox on your computer.
So, why would that inbox be “full”?
Two reasons come to mind:
One, the MP3 file you’re attempting to send may, itself, be bigger than the quota. For example if your ISP limits you to, say, 5 megabytes of email storage in your mail server’s inbox, then a 6 megabyte MP3 file simply won’t fit. In reality, because of the way attachments are encoded, it’s likely that a 4 megabyte, or even a 3 megabyte MP3 file also won’t fit. Attachments get bigger when they are encoded in email.
The other might be that your mail client could be configured to leave all messages on the server. This is typically not the default setting. What happens is that instead of moving your messages from the ISP’s mail server to your inbox, it copies them – leaving the originals on the server. There, they continue to accumulate and eventually your quota may be exceeded.
So the solution is to a) verify your mail quote with the ISP providing the account you’re attempting to receive the file on, and b) make sure that your email client is not configured to leave email on the server, but rather download it and remove it from the server.
And finally, if you’re just attempting to copy a file from one machine to another on the same network, mailing as an attachment is a lazy, inefficient way to do it. (Yes, I do it too, but it’s still lazy and inefficient.) The “right” way is to use file sharing and simply copy the file from one machine to the other. Much faster, and no quotas involved.