SMTP send attachment error

[GFYsweetwood]

I implemented the SMTP protocol.I can send text message,
but an error happened when I send the attachment.I use base64
encoding for attachment,the server can accept all of the data,
but the size of the attachment is smaller than the original,
the contents of the attachment is only part of the contents of the original.
Why?what's the reason?

[wizard_hu_]

Is the shortened attachment a part of the original one, or it is complete garbage?
Can it happen that you are specifying an explicit length field somewhere, which happens to be the length of the original data, but should be the length of the Base64-encoded variant?