I assume you have implemented the measures listed at How do I remove the restriction on port 25 from my Amazon EC2 instance or Lambda function?.
As per the error message
Error: 550 5.7.1 Unfortunately, messages from [my_IP_address] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140).
You EC2 IP is being blocked by receiving email server. Some companies may block emails from all EC2 IP addresses at part of their corporate security policy. Hence, changing of EC2 IP does not help. You will have to reach out to the organisation's security team to allow-list your EC2 IP.
Alternatively, you can explore using Amazon SES to send email. It supports SMTP interface. Under Pricing page, it states
Free tier customers receive up to 3,000 message charges free each month for the first 12 months after you start using SES. Do check Free Tier page for any updates on SES usage
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