There are two Options I know of for SMTP:
OPTION 1:
If you have Microsoft Exchange you can use Microsoft Direct Send
Here (Step 2)
You must have office 365
You will need your Office 365 Direct Send MX Value
The Value should look similar to - passwordreminder-cloud.mail.protection.outlook.com
This will require Port 25 to be opened to the internet for this to work.
To Locate your Direct Send Value
Log into Microsoft 365 Admin Center
Go to Settings > Domains, select your domain
Locate the MX Record
Copy that Record it should look similar to - passwordreminder-cloud.mail.protection.outlook.com
You should make sure you SPF record for your domain includes the (Public) IP Address of the server that is sending the email.
From Experience you may need to also configure a Connector from 365 to allow the emails to come through.
This may depend on your setup.
To do this:
Log into your Admin Exchange Center
Then go to mail flow, select Connectors
Add new connector
Enter a name for connector
Select Organization email server
For authentication you can use IP Address
You can enter the Public IP Address of the Server
**The above is not exact and every environment is different.
There may be some additional steps needed if the above doesn't work.
OPTION 2:
You can also use an third party service for SMTP
I would recommend SMTP2GO
As of this SMTP2GO does allow SMTP without Password Authentication (MUST USE PAID VERSION)
Information Here
I would suggest using there IP Authentication
Which allows you to enter the IP Address from where you are sending from and only that IP is allowed to send SMTP without authentication
Using this option you need to verify your domain using DNS Records.
** This is an Third Party Options and at any time SMTP2GO can change any of there pricing and options and this may stop working**