DBMail Attachment transfer failed

We are unable to send an excel file as attachment using dbmail.

EXEC msdb.dbo.sp_send_dbmail
@profile_name = 'Atlanta Automated Email Notification',
@recipients =  'myemailaddress@mydomain.gov',
@body = 'just testing',
@subject = 'e4345 Pending for PC5',
@file_attachments='\\s65da50\f$\midata\e4345\e4345pc5.xls';

The above script results in

"Msg 22051, Level 16, State 1, Line 0
Attachment transfer failed. Please see event log for more information."

However, we can send a different excel file with exact same script (just change file name to e4345pc1.xls) in the same folder with no problems (mail queued). Files have same permissions; only difference is that the first file is 1296 KB and the 2nd is 219 KB. However, the databasemail is configured for max file size of 7,000,000, well above 1296 KB.

Any ideas on how to get this to work?