What is the e-mail envelope? How is that different than the From: and To: headers?

An e-mail is significantly similar to a physical letter that you send in the mail. The envelope information for an e-mail is just like the information put on the outside of an envelope you send in the mail. A physical envelope usually contains two addresses, the sender's address, which may also be known as the "return address". The other address is known as the recipient's address. Please see: How to Label an Envelope: 13 Steps (with Pictures) - wikiHow for more information. The envelope of an e-mail also contains two groups of addresses, 1. The sender's (or return) e-mail address. 2. The recipients' addresses. The primary difference here is that an e-mail can have multiple recipient addresses.

When an envelope arrives at a Post Office, their systems inspect the recipient's address on the envelope to send it to the correct destination. The Post Office has no knowledge of the letter inside the envelope. That letter could contain addresses that have nothing to do with the addresses printed on the envelope. The envelope could say that the contents are for Adam, by the letter inside may be addressed to Eve. Or the envelope may be addressed to Adam, but the letter inside is addressed to both Adam and Eve.

For e-mails, the same thing is true. The envelope addresses determine how the e-mail is delivered, regardless of what content is displayed in the From:, To:, Cc:, and Bcc: headers. If the e-mail's envelope recipients contained both adam@example.com and eve@example.com, the message is then sent to both addresses, regardless of what the letter inside says the message is To:. This is why an e-mail that you receive may seem to be for some random person.


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We Answer Your Questions: FAQ

Q: What is the maximum e-mail attachment size?

A: The ETRN.com e-mail servers do not limit the size of individual e-mail attachments. The ETRN.com e-mail servers do impose a 400 MB maximum total message size limit. Individual customers can choose a smaller message size limit. We can also customize the handling of "over-sized" e-mails. Please contact us to discuss your specific needs. A couple of important facts:

1. Attachments are typically encoded in what is called Base64[1]. As a result, the actual length of MIME-compliant Base64-encoded binary data is usually about 137% of the original file size.

2. E-mails often contain both plain text and HTML components. This also increases the overall size of the e-mail.