If you want to setup an email service on your amazon ec2 instance you have to use Amazon SES.
The use of an own mail service is not possible because amazon does not want his servers to be misused for SPAM sendings.
To setup e-mail for your amazon ec2 instance follow these guides instead:
Integrating Amazon SES with Postfix
Amazon SES SMTP Issues
After that check your config by using sendmail and have a look at the log file
tail /var/log/mail.log
If you receive the following error for your amazon ec2 instance postfix mail setup (/var/log/mail.log):
postfix/smtp[3548]: F2A1845132: to=<xxxx@xxxx.de>, relay=none, delay=473, delays=473/0.02/0/0, dsn=4.4.3, status=deferred (Host or domain name not found. Name service error for name=email-smtp.eu-west-1.amazonaws.com type=AAAA: Host not found, try again) postfix/smtp[3548]: F2A1845132: to=<xxxx@xxxx.de>, relay=none, delay=473, delays=473/0.02/0/0, dsn=4.4.3, status=deferred (Host or domain name not found. Name service error for name=email-smtp.us-east-1.amazonaws.com type=AAAA: Host not found, try again) postfix/smtp[3548]: F2A1845132: to=<xxxx@xxxx.de>, relay=none, delay=473, delays=473/0.02/0/0, dsn=4.4.3, status=deferred (Host or domain name not found. Name service error for name=email-smtp.us-west-2.amazonaws.com type=AAAA: Host not found, try again)
then you have to disable ipv6 in your /etc/postfix/main.cf config
relayhost = [email-smtp.eu-west-1.amazonaws.com]:25 smtp_sasl_auth_enable = yes smtp_sasl_security_options = noanonymous smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd smtp_use_tls = yes smtp_tls_security_level = encrypt smtp_tls_note_starttls_offer = yes smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt inet_protocols = ipv4
Important is the last line (number 9)
If the domain still can not be found you have to update your resolv.conf for postfix:
sudo rm /var/spool/postfix/etc/resolv.conf sudo ln -s /etc/resolv.conf /var/spool/postfix/etc/resolv.conf
Restart the postfix service
sudo service postfix restart