trouble with sending mail with Spring mail through somain registrar 1and1 smtp

问题

I'm trying to send an email from a spring boot application with spring mail (1.5.8.RELEASE) JavaMailSender

sending through gmail SMTP is fine. however sending email through my domain registrar's amtp (registrar name- 1and1, in case someone already worked this out) is not working.

I'm getting this error, which imo - is more generic than useful: com.sun.mail.smtp.SMTPSenderFailedException: 550-Requested action not taken: mailbox unavailable 550 invalid DNS MX or A/AAAA resource record

domain I'm sending through is bank.org, and has this DNS settings:

bank.org.             3600    IN      A       178.79.161.205
bank.org.             3600    IN      MX      11 mx01.1and1.com.
bank.org.             3600    IN      MX      10 mx00.1and1.com.

the issue is, I'm able to send emails to gmail with the settings specified in this guide: http://www.baeldung.com/spring-email

these are settings to send with gmail:

spring.mail.host=smtp.gmail.com
spring.mail.port=587
spring.mail.username=<login user to smtp server>
spring.mail.password=<login password to smtp server>
spring.mail.properties.mail.smtp.auth=true
spring.mail.properties.mail.smtp.starttls.enable=true

However when i try to send messages via my registrar these settings are not successful:

spring.mail.host=smtp.1and1.com
spring.mail.port=587
spring.mail.username=username
spring.mail.password=password
spring.mail.properties.mail.smtp.auth=true
spring.mail.properties.mail.smtp.starttls.enable=true

thew issue is - I am able to connect to the 1and1 SMTP using thunderbird email client. so I know the credentials are working.

I guess what I need is to find out a way to detect the correct settings because the support team can't assist with issues related to sending email programmatically from spring mail