As a SMTP server, it must try hard to insure that all emails be sent to recipients successfully. But in fact, some mail service providers (like MSN, Hotmail, AOL, etc.) do not accept mail from some locations, worse, the mail is accepted and then going directly into the bin instead of the users mailbox. To solve this problem, in pervious version of Mail Express, we added a utility to let users add his/her ISP's or company's SMTP server into Mail Express Series as alternate SMTP server. When Mail Express send fail, it will try these alternate SMTP servers. But this method is really priggish. Mail Express always tried its own internet delivery engine first even if it had never sent successfully. It is inefficient.

From Mail Express Pro v4.0 and Lite 2.0, we updated its delivery engine. In these new versions, users can also add other public SMTP servers into Mail Express, but the difference is all these servers and Internal Delivery Engine are coordinative, you can change their order, let Mail Express always try other appointed SMTP first but its own Internal Delivery Engine, so much as remove Internal Delivery Engine in this delivery engine list.

As an advanced feature, it also supports rule, it means that users can customize the delivery. Users can add rule to make Mail Express send email to a special mail server via an appointed SMTP server bypass the delivery engines' order. For example, you can not send email to someone@aol.com, but you can send it out with SMTP server smtp.somehost.com, but smtp.somehost.com is not the first one in the delivery engine list, you can add a rule like this:

Usually, if we have an email account at an email service provider, and the SMTP server of this provider is very well, and we hope Mail Express can send email via the SMTP server of this provider when the from address is the account in this provider, you can also use rule to do this. For example, you have a account youraccount@someprovider.com in mail service provider someprovider.com whose SMTP server is smtp.someprovider.com, and you want Mail Express send email via smtp.someprovider.com when the from address is youraccount@someprovider.com, you can add a rule like this:

As another application, rule can also be used in this case: Because the Internal Delivery Engine will make a direct connection to the target mail server when Mail Express send email, so it is not fast and stable when you send a large email. In this case, perhaps you want Mail Express send it with your ISP's SMTP server or your company's SMTP server, you should use rule. For example, if you want Mail Express send email via you ISP's SMTP server smtp.someisp.com when it is bigger than 2MB, you should add a rule like this:

Now you can see, the delivery engine is very smart with high-performance, it can choose which SMTP server to use in different cases automatically and bypass the frequent problem. Mail Express is not only a great personal SMTP server with its own Internal Delivery Engine, but also a very useful SMTP Router with rule support, it can integrate your all SMTP servers into one delivery engine. Add your other SMTP servers into Mail Express when you can not send email to some addresses.