Getting 554 No SMTP service here error in Kubernetes

We have Azure Kubernetes service and out Master node is managed by Microsoft.

We created a dummy pod, and tried to send a test email using SSMTP but we got an error 554 No SMTP service here.

Steps used for sending an email :

kubectl run -it qatesting --image=debian -n monitoring
#apt-get update
#apt-get install ssmtp && apt-get install vim
#apt-get install sudo

apt-install ssmtp

#cd /etc/ssmtp/ssmtp.conf
#Vim ssmtp.conf
Edit mailhub=hostname and port 25
#touch mail.txt
#vim mail.txt
Subject: test email
Test text

sudo -u root ssmtp -v < mail.txt .

Error we get when performing this :

root@testingemail:/etc/ssmtp# sudo -u root ssmtp -v < mail.txt
[<-] 220 Hostname ESMTP Trustwave SEG Ready
[->] HELO testingemail
[<-] 554 No SMTP service here
ssmtp: 554 No SMTP service here