We tested this on Ubuntu 16x, 18x, 20x / Vesta CP /Hestia CP/ Exim4

Goto your mailjet dashboard

Take note of your Username (API Key) & Password (Secret Key)

Goto Mailjet senders list > Add whole domain names to approve the any-emials@your-domains What areas need to be touched on your server side ?

Only one File : /etc/exim4/exim4.conf.template

What options need to change ?

begin authenticators
send_via_mailjet
mailjet_smtp

Add the below additional entries into that file.

Under >> begin authenticators

mailjet_login:
driver = plaintext
public_name = LOGIN
client_send = : Username (API Key):Password (Secret Key)

Under >> begin routers

send_via_mailjet:
driver = manualroute
domains = ! +local_domains
transport = mailjet_smtp
route_list = * in.mailjet.com:587;

Under >> begin transports

mailjet_smtp:
driver = smtp
port = 587
hosts_require_auth = *
hosts_require_tls = *

Finally – Restart your exim4 / MTA

service exim4  restart