Notifications succeeds but return mail.bat script failed with exit code 1 (4046202)

When a NetVault notification is triggered, the following log error is returned:
Notification script "C:/Program Files (x86)/BakBone Software/NetVault Backup/scripts/users\mail.bat" failed:
'Script "C:/Program Files (x86)/BakBone Software/NetVault Backup/scripts/users\mail.bat" failed - exit code 1'

However the notification succeeds.

From the nvsendmailxxx.log trace file, we see the failure to find a value for the environement variable NVEMAIL1:
----------------------------
2 NVSENDMA:4108 2 0 170354 ReadArgs: parsing command line arguments
4 FILESYS :4108 32 0 170354 TRUE = FileSysOpen(0018FB8C, C:/Program Files (x86)/BakBone Software/NetVault Backup/config/mail.cfg, 101)
3 SYSTEM :4108 6 0 170354 SysGetEnvInteger failed
0 NVSENDMA:4108 14 0 170354 ReadNotifyEnvs(): error, env NVEMAIL1 unset
3 SYSTEM :4108 6 0 170354 SysGetEnvInteger failed
0 NVSENDMA:4108 5 0 170354 ReadArgs: could not load message
3 PROCESS :4108 13 0 170354 ProcCleanup(1)
--------------------------------