In a Windows 10 machine I'm using a tool (Netwrix Service Monitor) with a SMTP notification feature that doesn't support authentication, so I'm using Stunnel 5.72 as a local relay for SMTP2GO (smtp2go.com), with this configuration:

[smtp2go] client = yes accept = 127.0.0.1:25 connect = mail.smtp2go.com:2525 protocol = smtp protocolAuthentication = plain protocolHost = cloud.mydomain.com protocolUsername = [email protected] protocolPassword = P@ssw0rd debug = 7

I try both PLAIN and LOGIN as authentication mode, with this is error:

LOGIN: " ... TLS connected: new session negotiated TLSv1.3 ciphersuite: TLS_AES_256_GCM_SHA384 (256-bit encryption) Peer temporary key: X25519, 253 bits Compression: null, expansion: null -> HELO cloud.mydomain.com <- 250 mail.smtp2go.com Hello cloud.mydomain.com [x.x.x.x] -> AUTH LOGIN <- 503 AUTH command used when not advertised Remote server does not support LOGIN authentication ... "

PLAIN: " ... TLS connected: new session negotiated TLSv1.3 ciphersuite: TLS_AES_256_GCM_SHA384 (256-bit encryption) Peer temporary key: X25519, 253 bits Compression: null, expansion: null -> HELO cloud.mydomain.com <- 250 mail.smtp2go.com Hello cloud.mydomain.com [x.x.x.x] -> AUTH PLAIN ******************************************** <- 503 AUTH command used when not advertised PLAIN Authentication Failed Connection reset: 0 byte(s) sent to TLS, 0 byte(s) sent to socket Deallocating application specific data for session connect address ... "

Can anyone help me solve this problem? Thanks!