StringCat

A number of people uses more than one SMTP server. An SMTP server is the computer program, managed by your internet provider (employer, school, ISP,  …), that takes care of sending your email. Such an SMTP server can be a bare programs marginally implementing the standard RFC protocols, or it can be part of a larger server program, like the Microsoft Exchange Server.
 
More than one SMTP Server
There are good reasons for a user wanting to have more than one SMTP server available. Network servers go down occasionally. The time and the frequency they  faulter varies, but faulter they do. When your SMTP server is unavailable you cannot send your mission-critical email. As a safety net having a few extra SMTP servers at your disposal comes in quite handy.
 
Protection of SMTP servers
Another reason for having more freedom in picking out different SMTP server is the way they are protected. An SMTP server must be protected against “relaying” all email-send requests. If this protection would be absent the SMTP sever would immediately be used for relaying spam. There are basically three SMTP protections:

1. IP-mask: only sending from a certain subnet of internet addresses is allowed. This is typically what a large organization implements. The good thing is that it is transparent to the user, but the disadvantage is that a user not connected to the LAN of his employer cannot send email, unless he goes to the cumbersome and delaying procedure of setting up a VPN-connection
2. POP-first: a user has to login to his mailbox first. The email server records the IP-address of the POP service request and keeps it in its database for typically half an hour. If within that period a send command is requested from the same IP-address the SMTP server will fulfill the request. Email clients, like Outlook or Thunderbird, have to be configured to first login to the POP server.
3. SMTP-authentification: a login name and password is requested by the SMTP server. This authentification mechanism is not very popular. Yahoo uses this method, for instance

Any email cllient, including Outlook, allows to configure several SMTP servers. This extension introduces an ambiguity for the email client. If you want to send an email message the email client does not know which of the SMTP servers to use, unless you tell the program which one to use. This multiple choice is dangerous, because a wrong choice will lead to the failure of the sending of the email. If the email client uses the SMTP server that uses an IP-mask protection and you send the email message from an outise IP-address the relaying will fail. Users typically want to use the same email client wherever they are (office, home, notebook, ..), but its configuration – including the SMTP server settings – will differ from location to locations. Another feature is that users might want to use different “personalities”. In all those cases the SMTP-server the user wants to employ differ. 
 
Design flaw in Outlook
My email messages are regularly not being sent and are sitting sometimes a day in the outbox of Outlook. The reason is that Outlook has decided itself to pick out the wrong SMTP server from my list of SMTP servers. Even if I have indicated that another server is my “default” SMTP server. Outlook determines itself what SMTP server to use. In case the email to be send is a reply to another message Outlook decides to use the SMTP server connected to the account that holds the POP3 mailbox having received that earlier message. If Outlook can find in your list a Microsoft Exchange Server it will want to use that one, even if at that particular position it is not available. 
I gave up using Exchange Servers a long time ago. The unwanted synchronization between server and local machine took up so much of my computer resources that I couldn’t work any longer.
 
Request for change
If the user has indicated that a certain SMTP server is his default SMTP server, Outlook should be programmed to *always* use this server. Unless the user indicates explicitly that he wants to use a different SMTP server for this particular email message.

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