This might be a bug. Can you try the below and see if it works? If it does, then the bug was fixed in 11.20 SR6.
To enable username/password (ESMTP - AUTH LOGIN) authentication mode for the qvb.exe process add the following key to the settings.ini configuration file located in “C:\Windows\System32\config\systemprofile\AppData\Roaming\QlikTech\QlikViewBatch” and/or “C:\ProgramData\QlikTech\QlikViewBatch”:
SMTP_Authmethod = 2
SMTP_Authmethod = 0 (No authentication)
SMTP_Authmethod = 1 (CRAM MD5)
SMTP_Authmethod = 2 (AUTH LOGIN)
SMTP_Authmethod = 3 (AUTH PLAIN)
SMTP_Authmethod = 4 (NTLM
Unfortunately I will not be able to make this change on the production server where the issue is occurring. I will see if the same problem is manifesting itself on another server in that environment, and whether that config change fixes it.
I will post back when I make any progress.
Many thanks for your assistance.