Description: 
Upstream ArgoCD notification documentation simply states to use the RocketChat user's email for authentication.

[1] https://argocd-notifications.readthedocs.io/en/latest/services/rocketchat/

ArgoCD notifications is calling RocketChat API authentication endpoint /api/v1/login and according to RocketChat API reference, "If LDAP authentication is enabled, you must maintain the login in the same way as you normally do". 
Our RocketChat instance is configured to use sAMAccountName as login name. Generally, it is possible to configure RocketChat's LDAP to use email address instead of a username or accept both, but in our case, editing this configuration 
is not an option.  So the conclusion is that if RocketChat LDAP is configured to use LDAP username and not email, using the email address in the ArgoCD notifications service configuration will not work. 

RocketChat API reference: https://developer.rocket.chat/reference/api/rest-api/endpoints/other-important-endpoints/authentication-endpoints/login

RocketChat LDAP configuration examples:

https://docs.rocket.chat/use-rocket.chat/workspace-administration/settings/ldap/ldap-examples