Hello and thank you for choosing Lightsail!
As indicated in luis's answer, AWS does by default block traffic on port 25 initially. Lightsail instances have the same behavior and we have a guide to step you through the process: https://lightsail.aws.amazon.com/ls/docs/en_us/articles/amazon-lightsail-enabling-email-on-wordpress
I hope this helps!
AWS by default blocks the traffic on the port 25. Check-out this resource: https://aws.amazon.com/premiumsupport/knowledge-center/ec2-port-25-throttle/
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