Steps to reproduce:

1. Adding AWS SES smtp credentials.
2. Trigger an email alert
3. Check debug log

Result:

16252:20180723:082706.244 > MAIL FROM: >
16252:20180723:082706.266 < 501 Invalid MAIL FROM address provided
16252:20180723:082706.266 * MAIL failed: 501
16252:20180723:082706.266 > QUIT

Expected:
Email should be sent, it did work on Zabbix 3.0 LTS (we just updated to 4.0 alpha8)
SMTP credentials are valid, no firewall blocking, tested them on different application (same server)

With other SMTP Credentials (such as o365) it does the same empty MAIL FROM, but some providers are not so strict about that part since the username might supersede, not the case in Amazon's SES.