MailKit code works in development and staging but not in a Azure app service

• Problem: The code runs fine in Dev and staging environments but not after deployment to azure, to an app service. Since it’s connecting fine to the email server in the dev/staging environments I’m guessing its a configuration problem in Azure.

What I have done so far with no results:

• Changing the app service TLS versions in azure.
• Updated Mailkit.
• Checked the server certificate callback of the SmtpClient.
• Trying different account credentials.
• using port 587 or 465.
• Extending the timeout period.

Code (Fails on the connect method):

public async Task<SmtpClient> GetSmtpClient()
    {
        SmtpClient client = new();

        try
        {
            await client.ConnectAsync(smtpAddress, port, MailKit.Security.SecureSocketOptions.Auto);
            await client.AuthenticateAsync(username, password);
        }
        catch (Exception)
        {
            throw;
        }
        return client;
    }

Results in:

Exception: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.

StackTrace:
   at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.ThrowException(SocketError error, CancellationToken cancellationToken)
   at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.System.Threading.Tasks.Sources.IValueTaskSource.GetResult(Int16 token)
   at System.Threading.Tasks.ValueTask.ValueTaskSourceAsTask.<>c.<.cctor>b__4_0(Object state)
--- End of stack trace from previous location ---

At the end:

• I’m trying to get this to run on the azure app service.
• The exception occurs due to timeout when trying to connect to the email server which does not happen in dev/staging envs. So maybe the email server is not accepting connection from azure for some reason, resulting in socketError.

I really appreciate any clues you can give me. All the best.