How does your app send this files? Does it send HELO/EHLO identity? Perform SMTP AUTH?

I think the problem is how your application send out email. You must perform SMTP AUTH first, then send mail body/attachments.

I suggest you search in Google first to see how the SMTP related library work for your application. For example, PHP/Python/Java have their own smtp library, just check the sample code and you will get it work.

Hi Joe,

It's hard to say why it doesn't work for you without your smtp related code.
Port 465 uses SSL, but port 587 uses STARTTLS. They're different, and you must adjust your code to perform STARTTLS connection or SSL connection.

The main difference is, STARTTLS is a SMTP command you should execute after connection is established, then session is secure.
But with SSL, you have to establish a secure connection first, then execute all other SMTP commands.

I don't know which programming language you're using to write your application, so let me take Python for example here.
In Python smtplib module, there's a class "smtplib.SMTP_SSL" for SSL connection, and function "SMTP.starttls()" for STARTTLS connection.

As described in the module document:

And you cannot use them the same way. Here's simple example:

# SSL connection.
server = smtplib.SMTP_SSL('localhost')    # It uses port 465 by default.
server.login(...)
server.sendmail(...)
server.quit()

# STARTTLS connection.
server = smtplib.SMTP('localhost')
server.starttls()    # This is a SMTP command.
server.login(...)
server.sendmail(...)
server.quit()

Hope it helps.