I ran into an issue with Salesforce whereby Outbound Messages weren't being sent as the result of a Workflow rule.
I confirmed that the Workflow rule was being triggered correctly by adding an Email Alert to the workflow actions. (Thanks Manu)
The debug logs indicated the outbound rule was being triggered.
03:03:34.066 (66407000)|CODE_UNIT_STARTED|[EXTERNAL]|Workflow:01I50000000HxXS 03:03:34.088 (88062000)|WF_RULE_EVAL_BEGIN|Assignment 03:03:34.088 (88127000)|WF_RULE_EVAL_BEGIN|Response 03:03:34.088 (88153000)|WF_RULE_EVAL_BEGIN|Workflow 03:03:34.088 (88207000)|WF_CRITERIA_BEGIN|[XJob: a0gQ0000001UvXy a09P0000000yVcR]|X Job Configure on Insert|01Q50000000Mcd3|ON_CREATE_OR_TRIGGERING_UPDATE 03:03:34.092 (92958000)|WF_RULE_FILTER|[XJob : Notes contains run] 03:03:34.093 (93021000)|WF_RULE_EVAL_VALUE|null 03:03:34.093 (93062000)|WF_CRITERIA_END|false 03:03:34.093 (93117000)|WF_CRITERIA_BEGIN|[XJob: a0gQ0000001UvXy a09P0000000yVcR]|XJob Triggers by Page Count >0|01Q50000000Mcd8|ON_CREATE_OR_TRIGGERING_UPDATE 03:03:34.093 (93325000)|WF_RULE_FILTER|[XJob : Page Count greater than 0.0] 03:03:34.093 (93375000)|WF_RULE_EVAL_VALUE|0 03:03:34.093 (93406000)|WF_CRITERIA_END|false 03:03:34.093 (93455000)|WF_CRITERIA_BEGIN|[XJob: a0gQ0000001UvXy a09P0000000yVcR]|XJob Process XYZ|01Q50000000Mcks|ON_CREATE_OR_TRIGGERING_UPDATE 03:03:34.093 (93576000)|WF_RULE_FILTER|[XJob : Status equals Ready] 03:03:34.093 (93618000)|WF_RULE_EVAL_VALUE|2 03:03:34.093 (93640000)|WF_CRITERIA_END|true 03:03:34.095 (95547000)|WF_SPOOL_ACTION_BEGIN|Workflow 03:03:34.095 (95614000)|WF_RULE_INVOCATION|[XJob: a0gQ0000001UvXy a09P0000000yVcR] 03:03:34.095 (95641000)|WF_EMAIL_ALERT|Id=01WP0000000Cfow|CurrentRule:XJob Process XYZ (Id=01Q50000000Mcks) 03:03:34.204 (204879000)|WF_EMAIL_SENT|Template:00X30000000ltQt|Recipients:me@example.com |CcEmails: 03:03:34.204 (204966000)|WF_ACTION| Email Alert: 1; Outbound Message: 1; 03:03:34.204 (204997000)|WF_RULE_EVAL_BEGIN|Escalation 03:03:34.205 (205019000)|WF_RULE_EVAL_END 03:03:34.209 (209428000)|WF_ACTIONS_END| Email Alert: 1; Outbound Message: 1; 03:03:34.209 (209468000)|CODE_UNIT_FINISHED|Workflow:01I50000000HxXS
It turns out that the user who was triggering the Outbound Rule had a Profile where Administrative Permissions: Send Outbound Messages was unchecked. As a result the outbound message disappears without a trace (which would have been useful to see in the debug logs).
I've created an idea to see if a message can be logged when the outbound message is suppressed due to the users profile.
