3000AD Knowledge Base
Home » Categories » Knowledge Base Home » PDF-eXPLODE » General Knowledge

What is SMTP?


Please explain to me what SMTP stands for and how does it work? And why does PDF-eXPLODE not support IMAP?


All email protocols can be divided into OUTGOING email and INCOMING email protocols. 

The  OUTGOING email protocol  consists of one type only: SMTP.  

SMTP  stands for  S imple  M ail  T ransfer  P rotocol. 

SMTP is used to deliver emails from any email client, such as Outlook Express, MS Outlook or even PDF-eXPLODE to an email server or when an email is delivered from one email server to another. SMTP generally uses port 25. However a higher port no (like 465 or 587) is used for secured mail delivery. 

The  INCOMING email protocol  consists of 2 types:

POP3  - stands for  P ost  O ffice  P rotocol #3

POP3 allows an email client to download an email from an email server. The POP3 protocol is simple and does not offer many features except for download. Its design assumes that the email client downloads all available email from the server, deletes them from the server and then disconnects. POP3 normally uses port 110.

IMAP  - stands for  I nternet  M essage  A ccess  P rotocol

IMAP shares many similar features with POP3. It is also a protocol that is used to download email from an email server. However, IMAP has many more features than POP3. The IMAP protocol is designed to let users keep their email on the server. IMAP requires more disk space on the server and more CPU resources than POP3, as all emails are stored on the server. IMAP normally uses port 143. 

If you opened an email on one PC and read the email, the email client using POP3 will mark this as 'READ' - However if you should open the same email on another PC, then that copy of the email must be separately marked 'READ' by the email client again. There is no co-ordination of what emails have been read under the POP3 download process.  If however, your download email protocol used is IMAP, then the server synchronizes all copies from each source with the server email attributes - hence an email read on one PC will not be shown as unread if opened on another PC.

SMTP is what PDF-eXPLODE uses to send emails out of the server to recipients shown in the tag.  PDF-eXPLODE is only concerned with sending (Outgoing) emails and not receiving (Incoming) emails. Hence it does not need to support IMAP.

In PDF-eXPLODE an option called MAPI (MS Outlook) is also available. MAPI (Messaging Application Programming Interface) is a messaging architecture used to pass the Message in a format suitable for MS Outlook to on-send to the email recipient shown in the PDF-eXPLODE tag.

By default, the SMTP protocol works on one of four ports:

  • Port 25  - this is the default SMTP non-encrypted port
  • Port 2525  - this port is opened by some specific servers in case port 25 is filtered (by your ISP for example) and you want to send non-encrypted emails with SMTP
  • Port 465  - this is the port used, if you want to send encrypted messages using SMTP security protocol- SSL
  • Port 587 - this port must be used in conjunction with Security Protocol StartTLS. It first creates a secured connection, then encrypts the message before any sending occurs.


What is SSL and StartTLS security protocol?

When your email settings require you to use SSL (checked box in PDF-eXPLODE), the message and attachments (as 'data' packets) get encrypted and then sent. However when you choose StartTLS, this represents a security layer (or 'blanket') over the message sending process.  PDF-eXPLODE's sending engine will first create (via your SMTP server) a secure environment before it contacts the receiving server for sending. The sent data  still gets encrypted, but it is sent within a secured environment. So StartTLS is like an additional layer on top of the SSL encryption security. GMail as an SMTP server, is capable of sending email via Port 465/SSL checked as well as Port 587/StartTLS checked. 

Wherever your SMTP server is capable of these settings, it is preferable to send using Port 587/StartTLS checked; but the SMTP server must be capable of using those secure settings.  Here is a PDF-eXPLODE V4 email connection settings screen using Gmail 587 and 465 ports. Office365 uses the 587 Port/ StartTLS:















  You can get these email connection settings from your Internet/Email Service Provider or if you use MS Outlook, from the settings configuration.









SMTP is best explained with this YouTube video:

How are emails sent? For a very simple explanation of this highly technical process, you can view this 10 minute video: 


Comments Comments
There are no comments for this article. Be the first to post a comment.
Security Code Security Code

Subscribe to Knowledge Base

Get notified when new articles are added to the knowledge base.