The settings under "E-Mail Notification" are configured to use SMTP Authentication with SSL.

Using "Test configuration" leads to an exception: "java.net.SocketException: Connection closed by remote host". The full stack trace is attached.

This only happens with the "Test configuration" button. Regular build-failed mails actually work fine.

Mail is configured like this (see also jenkins_mail_config.png):

SMTP Server: smtp-relay.gmail.com
Use SMTP Authentication: checked
User Name and Password: entered
Use SSL: checked
SMTP Port: 465
Charset: UTF-8
Test e-mail recipient: e-mail address entered

Logging the traffic with sslsplit shows that AUTH LOGIN was not performed:

220 smtp-relay.gmail.com ESMTP h25sm4317wmb.21 - gsmtp
EHLO FIREBIRD
250-smtp-relay.gmail.com at your service, [_IP redacted_]
250-SIZE 157286400
250-8BITMIME
250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8
MAIL FROM:<redacted>
550-5.7.1 Invalid credentials for relay [_IP redacted_]. The IP address you've
550-5.7.1 registered in your G Suite SMTP Relay service doesn't match domain of
550-5.7.1 the account this email is being sent from. If you are trying to relay
550-5.7.1 mail from a domain that isn't registered under your G Suite account
550-5.7.1 or has empty envelope-from, you must configure your mail server
550-5.7.1 either to use SMTP AUTH to identify the sending domain or to present
550-5.7.1 one of your domain names in the HELO or EHLO command. For more
550-5.7.1 information, please visit
550 5.7.1 https://support.google.com/a/answer/6140680#invalidcred h25sm4317wmb.21 - gsmtp

Build-failed mails work as expected with AUTH LOGIN:

20 smtp-relay.gmail.com ESMTP cf12sm48397ejb.11 - gsmtp
EHLO FIREBIRD
250-smtp-relay.gmail.com at your service, [_IP redacted_]
250-SIZE 157286400
250-8BITMIME
250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8
AUTH LOGIN
334 VXNlcm5hbWU6
redacted
334 UGFzc3dvcmQ6
redacted
235 2.7.0 Accepted
MAIL FROM:<redacted>
250 2.1.0 OK cf12sm48397ejb.11 - gsmtp
RCPT TO:<redacted>
250 2.1.5 OK cf12sm48397ejb.11 - gsmtp
DATA
354 Go ahead cf12sm48397ejb.11 - gsmtp
Date: Sat, 17 Aug 2019 21:10:44 +0200 (CEST)
From: Jenkins <redacted>
To: redacted
Message-ID: <14698696.4.1566069044700.JavaMail.FIREBIRD$@FIREBIRD>
In-Reply-To: <21875298.1.1566064846550.JavaMail.FIREBIRD$@FIREBIRD>
References: <21875298.1.1566064846550.JavaMail.FIREBIRD$@FIREBIRD>
Subject: Build failed in Jenkins: Plakat.NET #1515
MIME-Version: 1.0
...