Hello StartTLS

output.txt

220 SG ESMTP service ready at ismtpd0008p1hnd1.sendgrid.net

[EHLO]
250-smtp.sendgrid.net
250-8BITMIME
250-PIPELINING
250-SIZE 31457280
250-STARTTLS
250-AUTH PLAIN LOGIN
250 AUTH=PLAIN LOGIN

[STARTTLS]
220 Begin TLS negotiation now

[Upgrade socket]

[EHLO (tls)]
250-smtp.sendgrid.net
250-8BITMIME
250-PIPELINING
250-SIZE 31457280
250-STARTTLS
250-AUTH PLAIN LOGIN
250 AUTH=PLAIN LOGIN

[QUIT (tls)]
221 See you later

test.ps1

$sock = [Net.Sockets.TcpClient]::new('smtp.sendgrid.net', 587)
$tx = [Io.StreamWriter]::new($sock.GetStream())
$rx = [Io.StreamReader]::new($sock.GetStream())
$tx.AutoFlush = $true

$rx.ReadLine()

""
"[EHLO]"
$tx.WriteLine('EHLO smtp.sendgrid.net')
do {
    $r = $rx.ReadLine()
    $r
} while ($r[3] -eq '-')

""
"[STARTTLS]"
$tx.WriteLine('STARTTLS')
$rx.ReadLine()

""
"[Upgrade socket]"
$ssl = [Net.Security.SslStream]::new($sock.GetStream(), $false, $null, $null)
$ssl.AuthenticateAsClient('smtp.sendgrid.net')
$tx = [Io.StreamWriter]::new($ssl)
$rx = [Io.StreamReader]::new($ssl)
$tx.AutoFlush = $true

""
"[EHLO (tls)]"
$tx.WriteLine('EHLO smtp.sendgrid.net')
do {
    $r = $rx.ReadLine()
    $r
} while ($r[3] -eq '-')

""
"[QUIT (tls)]"
$tx.WriteLine('QUIT')
$rx.ReadLine()