Using phpBB with SparkPost mail service, Send a test email button bug

i am having trouble sending mail through SparkPost Email Delivery Service https://www.sparkpost.com/

in ACP -> General tab -> Client communication -> E-mail settings.
The Send a test email button is broken when using the SparkPost Email Delivery Service as SMTP info

i tested the Send a test email button and i get a error in phpbb logs

but if i use the Mass email or send mail by the users profile all emails works and have no email error log in phpbb so there is a bug with the The email test button.

so I disabled the functionality that obscures private information from the logs and there it was. It looks like the acp/email page is feeding the value of the password field (which is ********) into $config. SparkPost is understandably miffed when it receives a bunch of asterisks instead of a real password and promptly refuses service. this only happens when using the Send a test email button

some other ppl have the same prob : https://www.phpbb.com/community/viewtopic.php?f=556&t=2397786

EMAIL ERROR LOG:

Email error
» EMAIL/SMTP
/adm/index.php

Ran into problems sending email at Line 1553. Response: 535 5.7.8 Sorry.
.
Backtrace
Connecting to smtp.sparkpostmail.com:587
LINE: 1096 <- 220 2.0.0 smtp.sparkpostmail.com ESMTP ecelerity 4.2.29.58592 r(Core:4.2.29.1) Wed, 25 Jan 2017 02:07:24 +0000

1. EHLO vps102464.vps.ovh.ca
   LINE: 1443 <- 250-momentum1.platform1.us-west-2.aws.cl.messagesystems.com says EHLO to 149.56.130.51:35566

LINE: 1443 <- 250-ENHANCEDSTATUSCODES

LINE: 1443 <- 250-PIPELINING

LINE: 1443 <- 250-8BITMIME

LINE: 1443 <- 250-AUTH=LOGIN PLAIN

LINE: 1443 <- 250-AUTH LOGIN PLAIN

LINE: 1443 <- 250 STARTTLS

1. STARTTLS
   LINE: 1488 <- 220 2.0.0 continue

1. EHLO vps102464.vps.ovh.ca
   LINE: 1443 <- 250-momentum1.platform1.us-west-2.aws.cl.messagesystems.com says EHLO to 149.56.130.51:35566

LINE: 1443 <- 250-ENHANCEDSTATUSCODES

LINE: 1443 <- 250-PIPELINING

LINE: 1443 <- 250-8BITMIME

LINE: 1443 <- 250-AUTH=LOGIN PLAIN

LINE: 1443 <- 250 AUTH LOGIN PLAIN

1. AUTH PLAIN
   LINE: 1546 <- 334

1. Omitting sensitive information
   LINE: 1553 <- 535 5.7.8 Sorry.