By default, OnTime uses a built-in SMTP client to send out e-mail notifications.  The advantage of this is that it does not rely on any 3rd party SMTP library and it's very light-weight, but the disatvantage is that it's not as configurable as some SMTP servers may require it to be. 

The .Net framework also has built-in support for sending e-mails through the System.Web.Mail namespace, but it relies on the CDOSYS component.  Some systems may not necessarily have CDO installed by default which makes using it less desirable.

OnTime 2005 uses pluggable assemblies for the SMTP code so that it does not rely on a particular implementation to send out e-mail notifications.  OnTime ships with both the built-in support for SMTP, as well as a CDO implementation, and by default it uses the built-in SMTP code so that it does not have to rely on CDO being installed.  Each implementation is contained in its own assembly, and these assemblies are installed in the \Plugins\Mail subfolder of the main installation folder.  Which assembly OnTime uses is controlled through a value of a column in one of the tables in the database.  That is the 'SmtpAssembly' field in the 'NotificationSettings' table.

So, if you are having problems with e-mail notifications with the default implementation, you may try switching to the CDO implementation (assuming you have CDO installed) by going into the 'NotificationSettings' table in the OnTime database and modifying the 'SmtpAssembly' field to the appropriate assembly name: 'Axosoft.Common.CDO.dll'.  The default value in this field should be 'Axosoft.Common.Smtp.dll'.  You will have to restart OnTime for the change to take place.