AWS SeS From Lambda (works with Python, not with .Net) - amazon-web-services

Question

I have a .NET lambda that sends emails via SES. I have a VPCe for SeS coupled with all of the required port, subnet & ACL config.
I created a Python lambda to do exactly the same as my .NET Lambda. It's joined to the same subnets, same security group & uses the same SMTP user and password.
The Python code works fine, connects to SES and sends the emails without fail but my .NET Lambda doesn't.
The last thing in my logs is:
ConnectAsync is going to be called
When calling the uri/path it says "Service Unavailable, 500."
I know the code works as if it's in debug on my dev machine pointing at the www-ses endpoint, it works fine.
I've also tried Nat gateways as an alternative (same issue) but I've settled with VPCe as I know my Python lambda works fine.
.NET:
try
{
using var smtp = new SmtpClient();
LambdaLogger.Log($"smtp ConnectAsync is going to be called for host {Configuration["SmtpHost"]} and port {Configuration["SmtpPort"].To<int>()} ");
//await smtp.ConnectAsync(Configuration["SmtpHost"], Configuration["SmtpPort"].To<int>(), SecureSocketOptions.StartTlsWhenAvailable);
await smtp.ConnectAsync("vpce-0af6f6a54b8c6d652-ltbm2mcx.email-smtp.eu-west-2.vpce.amazonaws.com", 587, SecureSocketOptions.StartTlsWhenAvailable);
LambdaLogger.Log($"SMTP connection created on {Configuration["SmtpHost"]} and port {Configuration["SmtpPort"].To<int>()}.");
await smtp.AuthenticateAsync(Configuration["SESUserName"], Configuration["SESPassword"]);
LambdaLogger.Log($"User auth completed for user .");
await smtp.SendAsync(email);
LambdaLogger.Log("email sent.");
emailMessage.isDeleted = true;
_applicationContext.EmailMessages.Update(emailMessage);
smtp.Disconnect(true);
}
catch (Exception ex)
{
emailMessage.Retries = emailMessage.Retries++;
_applicationContext.EmailMessages.Update(emailMessage);
LambdaLogger.Log($"Email Sending Error: {ex.ToFullMessage()}");
}
await _applicationContext.SaveChangesAsync();
Python:
import json
import smtplib
from email.mime.text import MIMEText
def lambda_handler(event, context):
sender = '<removd>'
receivers = ['<removed>']
port = 587
msg = MIMEText('This is test mail')
msg['Subject'] = 'Test mail'
msg['From'] = '<removed>'
msg['To'] = '<remvoed>'
with smtplib.SMTP('vpce-0af6f6a54b8c6d652-ltbm2mcx.email-smtp.eu-west-2.vpce.amazonaws.com', 587) as server:
server.starttls()
server.login('<removed>', '<removed>')
server.sendmail(sender, receivers, msg.as_string())
print("Successfully sent email")