Which provider are you using for your SMTP?
Services
Office 365?
Gmail?
Internal Relay?
Mimecast?
…
Information
You will be required to add an exception or a rule in place to allow multiple emails as these services will block your connection if you’re firing to many emails.
My suggestion is to setup an internal relay email server (barebox)
Sequence
App Server → Internal Relay (172.168.1.x :: 25) → Office 365, Gmail
Guide
Here is a guide: Multilple E-mails without anonymous authentication - Experts’ Corner - Epicor User Help Forum (epiusers.help)