[Android Apps and Games] M66B:
I have no idea why mailbox.org doesn't accept a login:

With LOGIN authentication (default):

08-13 14:15:16.127 9091 9171 I System.out: DEBUG SMTP: useEhlo true, useAuth true
08-13 14:15:16.127 9091 9171 I System.out: DEBUG SMTP: trying to connect to host "smtp.mailbox.org", port 465, isSSL true
08-13 14:15:16.612 9091 9171 I System.out: 220 smtp1.mailbox.org ESMTP Postfix
08-13 14:15:16.612 9091 9171 I System.out: DEBUG SMTP: connected to host "smtp.mailbox.org", port: 465
08-13 14:15:16.615 9091 9171 I System.out: EHLO localhost
08-13 14:15:16.636 9091 9171 I System.out: 250-smtp1.mailbox.org
08-13 14:15:16.636 9091 9171 I System.out: 250-PIPELINING
08-13 14:15:16.636 9091 9171 I System.out: 250-SIZE 143699726
08-13 14:15:16.637 9091 9171 I System.out: 250-ETRN
08-13 14:15:16.637 9091 9171 I System.out: 250-AUTH PLAIN LOGIN
08-13 14:15:16.637 9091 9171 I System.out: 250-AUTH=PLAIN LOGIN
08-13 14:15:16.637 9091 9171 I System.out: 250-ENHANCEDSTATUSCODES
08-13 14:15:16.637 9091 9171 I System.out: 250-8BITMIME
08-13 14:15:16.637 9091 9171 I System.out: 250 DSN
08-13 14:15:16.637 9091 9171 I System.out: DEBUG SMTP: Found extension "PIPELINING", arg ""
08-13 14:15:16.637 9091 9171 I System.out: DEBUG SMTP: Found extension "SIZE", arg "143699726"
08-13 14:15:16.637 9091 9171 I System.out: DEBUG SMTP: Found extension "ETRN", arg ""
08-13 14:15:16.637 9091 9171 I System.out: DEBUG SMTP: Found extension "AUTH", arg "PLAIN LOGIN"
08-13 14:15:16.637 9091 9171 I System.out: DEBUG SMTP: Found extension "AUTH=PLAIN", arg "LOGIN"
08-13 14:15:16.637 9091 9171 I System.out: DEBUG SMTP: Found extension "ENHANCEDSTATUSCODES", arg ""
08-13 14:15:16.637 9091 9171 I System.out: DEBUG SMTP: Found extension "8BITMIME", arg ""
08-13 14:15:16.637 9091 9171 I System.out: DEBUG SMTP: Found extension "DSN", arg ""
08-13 14:15:16.638 9091 9171 I System.out: DEBUG SMTP: protocolConnect login, host=smtp.mailbox.org, user=user@mailbox.org, password=<non-null>
08-13 14:15:16.638 9091 9171 I System.out: DEBUG SMTP: Attempt to authenticate using mechanisms: LOGIN PLAIN DIGEST-MD5 NTLM XOAUTH2
08-13 14:15:16.638 9091 9171 I System.out: DEBUG SMTP: Using mechanism LOGIN
08-13 14:15:16.638 9091 9171 I System.out: DEBUG SMTP: AUTH LOGIN command trace suppressed
08-13 14:15:18.803 9091 9171 I System.out: DEBUG SMTP: AUTH LOGIN failed
08-13 14:15:18.809 9091 9171 E eu.faircode.email: javax.mail.AuthenticationFailedException: 535 5.7.8 Error: authentication failed: password

With PLAIN authentication (forced):

08-13 14:16:27.987 9284 9351 I System.out: DEBUG SMTP: useEhlo true, useAuth true
08-13 14:16:27.987 9284 9351 I System.out: DEBUG SMTP: trying to connect to host "smtp.mailbox.org", port 465, isSSL true
08-13 14:16:28.151 9284 9351 I System.out: 220 smtp1.mailbox.org ESMTP Postfix
08-13 14:16:28.151 9284 9351 I System.out: DEBUG SMTP: connected to host "smtp.mailbox.org", port: 465
08-13 14:16:28.153 9284 9351 I System.out: EHLO localhost
08-13 14:16:28.175 9284 9351 I System.out: 250-smtp1.mailbox.org
08-13 14:16:28.175 9284 9351 I System.out: 250-PIPELINING
08-13 14:16:28.175 9284 9351 I System.out: 250-SIZE 143699726
08-13 14:16:28.175 9284 9351 I System.out: 250-ETRN
08-13 14:16:28.175 9284 9351 I System.out: 250-AUTH PLAIN LOGIN
08-13 14:16:28.175 9284 9351 I System.out: 250-AUTH=PLAIN LOGIN
08-13 14:16:28.175 9284 9351 I System.out: 250-ENHANCEDSTATUSCODES
08-13 14:16:28.175 9284 9351 I System.out: 250-8BITMIME
08-13 14:16:28.175 9284 9351 I System.out: 250 DSN
08-13 14:16:28.177 9284 9351 I System.out: DEBUG SMTP: Found extension "PIPELINING", arg ""
08-13 14:16:28.177 9284 9351 I System.out: DEBUG SMTP: Found extension "SIZE", arg "143699726"
08-13 14:16:28.177 9284 9351 I System.out: DEBUG SMTP: Found extension "ETRN", arg ""
08-13 14:16:28.177 9284 9351 I System.out: DEBUG SMTP: Found extension "AUTH", arg "PLAIN LOGIN"
08-13 14:16:28.178 9284 9351 I System.out: DEBUG SMTP: Found extension "AUTH=PLAIN", arg "LOGIN"
08-13 14:16:28.179 9284 9351 I System.out: DEBUG SMTP: Found extension "ENHANCEDSTATUSCODES", arg ""
08-13 14:16:28.179 9284 9351 I System.out: DEBUG SMTP: Found extension "8BITMIME", arg ""
08-13 14:16:28.179 9284 9351 I System.out: DEBUG SMTP: Found extension "DSN", arg ""
08-13 14:16:28.179 9284 9351 I System.out: DEBUG SMTP: protocolConnect login, host=smtp.mailbox.org, user=user@mailbox.org, password=<non-null>
08-13 14:16:28.179 9284 9351 I System.out: DEBUG SMTP: Attempt to authenticate using mechanisms: PLAIN LOGIN DIGEST-MD5 NTLM
08-13 14:16:28.179 9284 9351 I System.out: DEBUG SMTP: Using mechanism PLAIN
08-13 14:16:28.179 9284 9351 I System.out: DEBUG SMTP: AUTH PLAIN command trace suppressed
08-13 14:16:29.768 9284 9351 I System.out: DEBUG SMTP: AUTH PLAIN failed
08-13 14:16:29.770 9284 9351 E eu.faircode.email: javax.mail.AuthenticationFailedException: 535 5.7.8 Error: authentication failed:

Maybe external access needs to be enabled?