Report Subscription fails, shows 'success', specific scenario

I have a report that will run successfully except for one very specific scenario.  Any help or suggestions tracking down this issue would be helpful.

The report is named ProjectsWeeklyReport.rdl and takes a single parameter of a specific user's login account "DOMAIN\username"

• The report runs successfully in the web regardless of the parameter  (works for all users, including john.doe)
• The report runs successfully in Report Builder regardless of the parameter (works for all users, including john.doe)
• The report runs successfully as a subscription with the report included as aMHTML file and a statically-selected parameter EXCEPT for john.doe.  The same report works for all other users selected as the parameter
• The report runs successfully as a subscription with the report included as aExcel file (instead of an MHTML file) regardless of the statically-selected parameter (works for all users, including john.doe)

To summarize, in order for the report subscription email to fail, these conditions have to be met (otherwise the result is success):
 - Report must have the parameter "DOMAIN\john.doe"
 - Report must NOT be MHTML format in the email

I can see the subscribed report worked according to the ReportExecutionLog in MS SQL.  Is there any reason why the MHTML version of the report subscription would fail to email?  Any reason why it would fail as MHTML but be fine in other formats (PDF & Excel both tested success)

For example, this one was never emailed:

InstanceName:  APPSERVER\@Sharepoint 
ItemPath:  /{73419db6-f0e2-4096-a710-c630e8a26295}/Reports/ProjectsWeeklyReport.rdl 
Username:  DOMAIN\p_sps_app_pool_porta   (App pool account)
Execution ID:  rkciwraxt5cmoi55niw3wx45
RequestType:  Subscription 
Format:  MHTML 
Parameters:   Engineer=DOMAIN%5Cjohn.doe
ItemAction:  Render 
TimeStart:  7/22/2014 4:27:05 PM
TimeEnd:   7/22/2014 4:27:12 PM
TimeDataRetrieval:  1200
TimeProcessing:  75
TimeRendering:   5667
Source:  Live 
Status:  rsSuccess 
ByteCount:  277050 
RowCount:  431 

NOTES:

• There are no logs in Exchange message tracking of this email ever reaching our servers
• The Manage Subscriptions page shows the report emailed successfully  "Email sent to.." with the correct timestamp of 7/22 at 4:27 PM

And compare to this one that was successfully emailed:

InstanceName:  APPSERVER\@Sharepoint 
ItemPath:  /{73419db6-f0e2-4096-a710-c630e8a26295}/Reports/ProjectsWeeklyReport.rdl 
Username:  DOMAIN\p_sps_app_pool_porta   (App pool account)
Execution ID:  1yycmuiwc4hzymzuyj1zbc45
RequestType:  Subscription 
Format:  EXCELOPENXML
Parameters:   Engineer=DOMAIN%5Cjohn.doe
ItemAction:  Render 
TimeStart:  7/22/2014 4:21:10 PM
TimeEnd:   7/22/2014 4:21:18 PM
TimeDataRetrieval:  1574 
TimeProcessing:  73 
TimeRendering:   5912
Source:  Live 
Status:  rsSuccess 
ByteCount:  34180
RowCount:  431 

Suggestions of where to look for errors? Troubleshooting ideas?  Anything would be helpful!  Thanks in advance.