If you install Mail in a Box or ANY email server on AWS Lightsail you will be unable to send emails to Microsoft email accounts including at outlook.com and hotmail.com
This is due to a war between MS and AWS by which ALL of the IP addresses at AWS are listed as on the spam blacklist (Level 3) by the spam checking company that Microsoft uses to decide whether to allow emails through. The effect is Microsoft silently drops all emails from any email server on AWS lightsail instance.
If you pay for support from AWS, you will find that AWS state clearly that they have not been able to resolve this with the spam checking company who want AWS to pay $50 for every IP address and this must be done all at once not one IP address at a time.
The advice I got from AWS was to transfer my email servers off AWS and onto a different provider who offers IPs that are not on the spam list.
A challenge is that it is also necessary to find an ISP that allows you to change IP address (in case the one they offer you is already registered on spam blacklists) and that they open port 25.
Perhaps someone here on this forum can offer advice on a good ISP?
Best wishes,
Terry