How did you converted the stream to base64?

Julian is correct, your code is inserting line breaks, remove the lines:

    If 'pInput.AtEnd {
        Set tSC = tStream.Write($C(10))
        $$$TRACE("Adding terminator.")
    }

Since it's a vmware issue, not an InterSystems issue, I think you should ask vmware (now Broadcom).

What's the status returned by the Send() method of %Net.SMTP class?
I believe it should contain some info.

Another option in one line:

do ##class(%SYS.Namespace).ListFunc().%Display()

What operating system? What InterSystem product? What Version?

Your Business Operation (is supposed to) use the EnsLib.EMail.OutboundAdapter, but the in the code you use the %Net.SMTP class directly, ignoring the adapter.

In the Interoperability portal your implementation  provide the email adapter configuration (server, certificate, credentials etc. etc.) that in fact are then ignored in the code. This makes it very confusing for anyone using the portal, now...and in the future!

So the first question is, do you want to use the EnsLib.EMail.OutboundAdapter or not?

If you want to use the adapter, if you have not already done it, I suggest to start with the documentation:

Using the Email Outbound Adapter and Settings for the Email Outbound Adapter

If you don't what to use the EnsLib.EMail.OutboundAdapter, then remove the reference from your code.

Using %Net.SMTP class directly (right?), you said that "...I get the error that the property UseSTARTTLS is not available." and this is very strange, can you confirm you are using IRIS version 2021.1?
Is your code the same as in your first post or has changed? If changed, can you post the modified code that fail?

Last but not least, are you sure your outlook.com account allow user/password authentication? Or you must use OAuth2 authentication??

From Stream Compression documentation:

.....new stream data is automatically compressed, except for the following cases, where the data is not suitable for compression:

• The stream can be stored in a single chunk less than 1024 characters long.
• The stream is already compressed, that is, the first chunk of data matches a typical compressed file format, such as JPEG, MP3, or ZIP.
• The first 4KB of the stream cannot be compressed by at least 20 percent.

I guess your test runs into these cases.

What's your use case? Are you using Ens.BusinessProcess or Ens.BusinessProcessBPL?

To call a class method the syntax is:

##class(ClassName).MethodName(params)

In the documentation page there is a "Feedback" button/link on the right, I encourage you to provide your comments as Feedback, possibly linking this post in the Community.

Hi Timo, your sample is not a valid/full check for "valid number":

USER>set number="123.40"
 
USER>write (+number=number)
0
USER>write $isvalidnum(number)
1