In general, these two terms are used to refer to the email addresses of the sender and recipients specified in the email, or during the email transmission. SMTP and MIME email addresses in most cases are identical. The following representation shows the basic commands used to send an email message using the SMTP protocol and highlights the difference between the SMTP and MIME email addresses.
From the above, we can see that the sender's email address is specified two times once in the mail from SMTP command, and another time in the FROM .
The same occurs for the recipient's email address: It is first specified in the rcpt to SMTP command, and then it is specified in the TO.
The SMTP email addresses are only used by SMTP servers during the transmission of the email to route the message to destination. This information is normally lost when the email is saved in the recipient's mailbox.
On the other hand, the MIME addresses are addresses specified within the FROM: and the TO: fields. These fields are part of the message being transferred, and they are both specified after the data SMTP command. This information is normally not utilized by the SMTP servers transmitting the email. However, the MIME email addresses are the email addresses which are displayed to the user.
Spam emails sometimes specify different SMTP email addresses and MIME email addresses. This may be done on purpose by spammers in an attempt to fool Anti-Spam software, or the user. It may also be the result of a mistake or a bug in the software used by the spammer to send his spam emails. The following diagram illustrates such an attempt:
- When transmitting an email, only the email address in the rcpt to command needs to be real, since this specifies the email address where the email will be delivered. All the other email addresses can be fake.
- The email sent using the second SMTP transmission above, will be displayed to the recipient as follows:
To: you [firstname.lastname@example.org]
Subject: This is the message subject
This is the message body
- GFI MailEssentials has access to the SMTP information, since it is bound to the Microsoft Internet Information Services (IIS) SMTP server. GFI MailEssentials will also have access to a copy of the message, which will have the MIME information.