Mailhog can be accessed via ports {{ .Values.service.port.http }} (HTTP) and {{ .Values.service.port.smtp }} (SMTP) on the following DNS name from within your cluster:
{{ include "mailhog.fullname" . }}.{{ .Release.Namespace }}.svc.cluster.local

If you'd like to test your instance, forward the ports locally:

Web UI:
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export POD_NAME=$(kubectl get pods --namespace {{ .Release.Namespace }} -l "app.kubernetes.io/name={{ include "mailhog.name" . }},app.kubernetes.io/instance={{ .Release.Name }}" -o jsonpath="{.items[0].metadata.name}")
kubectl port-forward --namespace {{ .Release.Namespace }} $POD_NAME 8025

SMTP Server:
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export POD_NAME=$(kubectl get pods --namespace {{ .Release.Namespace }} -l "app.kubernetes.io/name={{ include "mailhog.name" . }},app.kubernetes.io/instance={{ .Release.Name }}" -o jsonpath="{.items[0].metadata.name}")
kubectl port-forward --namespace {{ .Release.Namespace }} $POD_NAME 1025