figure - avoid using hard-coded values for the local system whenever possible defining the parts of the letter the "envelopefor an -mail address is what the smtp server uses to route the -mail howeverthe envelope doesn' include any content -that' the purpose of the letter lot of developers get the two elements confused because the letter contains sender and receiver information as well this information appears in the letter just like the address information that appears in business letter -it' for the benefit of the viewer when you send business letterthe postal delivery person doesn' open the envelope to see the address information inside only the information on the envelope matters it' because the information in the -mail letter is separate from its information in the envelope that nefarious individuals can spoof -mail addresses the envelope potentially contains legitimate sender informationbut the letter may not (when you see the -mail in your -mail applicationall that is present is the letternot the envelope -the envelope has been stripped away by the -mail application for that matterneither the sender nor the recipient information may be correct in the letter that you see onscreen in your -mail reader the letter part of an -mail is actually made of separate componentsjust as the envelope is here is summary of the three componentssenderthe sender information tells you who sent the message it contains just the -mail address of the sender receiverthe receiver information tells you who will receive the message this is actually list of recipient -mail addresses even if you want to send the message to only one personyou must supply the single -mail address in list