Wordpress E-Mail Funktion - 421 invalid sender domain

Hallo,

ich verwende Wordpress mit dem Plugin WP-Mail-SMTP. Ich habe ein bestehendes SMTP Postfach bei 1und1.

Beim versenden einer Testmail, bekomme ich folgende Meldung:

bool(false)
The full debugging output is shown below:
object(PHPMailer)#157 (69) {
["Version"]=>
string(5) "5.2.7"
["Priority"]=>
int(3)
["CharSet"]=>
string(5) "UTF-8"
["ContentType"]=>
string(10) "text/plain"
["Encoding"]=>
string(4) "8bit"
["ErrorInfo"]=>
string(214) "The following From address failed: wordpress@192.168.16.54 : MAIL FROM command failed,421,invalid sender domain '192.168.16.54' (misconfigured dns?) http://postmaster.1and1.com/error-messages/#invalidsenderdomain
"

The SMTP debugging output is shown below:
SMTP -> get_lines(): $data was ""SMTP -> get_lines(): $str is "220 smtp.1und1.de (mreue003) Welcome to Nemesis ESMTP server
"SMTP -> get_lines(): $data is "220 smtp.1und1.de (mreue003) Welcome to Nemesis ESMTP server
"SMTP -> FROM SERVER:220 smtp.1und1.de (mreue003) Welcome to Nemesis ESMTP server
CLIENT -> SMTP: EHLO 192.168.16.54
SMTP -> get_lines(): $data was ""SMTP -> get_lines(): $str is "250-smtp.1und1.de
"SMTP -> get_lines(): $data is "250-smtp.1und1.de
"SMTP -> get_lines(): $data was "250-smtp.1und1.de
"SMTP -> get_lines(): $str is "250-STARTTLS
"SMTP -> get_lines(): $data is "250-smtp.1und1.de
250-STARTTLS
"SMTP -> get_lines(): $data was "250-smtp.1und1.de
250-STARTTLS
"SMTP -> get_lines(): $str is "250-AUTH LOGIN PLAIN
"SMTP -> get_lines(): $data is "250-smtp.1und1.de
250-STARTTLS
250-AUTH LOGIN PLAIN
"SMTP -> get_lines(): $data was "250-smtp.1und1.de
250-STARTTLS
250-AUTH LOGIN PLAIN
"SMTP -> get_lines(): $str is "250-AUTH=LOGIN PLAIN
"SMTP -> get_lines(): $data is "250-smtp.1und1.de
250-STARTTLS
250-AUTH LOGIN PLAIN
250-AUTH=LOGIN PLAIN
"SMTP -> get_lines(): $data was "250-smtp.1und1.de
250-STARTTLS
250-AUTH LOGIN PLAIN
250-AUTH=LOGIN PLAIN
"SMTP -> get_lines(): $str is "250-SIZE 120000000
"SMTP -> get_lines(): $data is "250-smtp.1und1.de
250-STARTTLS
250-AUTH LOGIN PLAIN
250-AUTH=LOGIN PLAIN
250-SIZE 120000000
"SMTP -> get_lines(): $data was "250-smtp.1und1.de
250-STARTTLS
250-AUTH LOGIN PLAIN
250-AUTH=LOGIN PLAIN
250-SIZE 120000000
"SMTP -> get_lines(): $str is "250 HELP
"SMTP -> get_lines(): $data is "250-smtp.1und1.de
250-STARTTLS
250-AUTH LOGIN PLAIN
250-AUTH=LOGIN PLAIN
250-SIZE 120000000
250 HELP
"SMTP -> FROM SERVER:250-smtp.1und1.de
250-STARTTLS
250-AUTH LOGIN PLAIN
250-AUTH=LOGIN PLAIN
250-SIZE 120000000
250 HELP
CLIENT -> SMTP: AUTH LOGIN
SMTP -> get_lines(): $data was ""SMTP -> get_lines(): $str is "334 ADFkrl9eimlO9
"SMTP -> get_lines(): $data is "334 ADFkrl9eimlO9
"SMTP -> FROM SERVER:334 ADFkrl9eimlO9
CLIENT -> SMTP: x3lraS8uby1yIXBseUBiZAx0aW9zLmNvbS==
SMTP -> get_lines(): $data was ""SMTP -> get_lines(): $str is "334 ADFkrl9eimlO9
"SMTP -> get_lines(): $data is "334 ADFkrl9eimlO9
"SMTP -> FROM SERVER:334 ADFkrl9eimlO9
CLIENT -> SMTP: YmVabGlvc18zPDk4X2dpa7k=
SMTP -> get_lines(): $data was ""SMTP -> get_lines(): $str is "235 Authentication successful
"SMTP -> get_lines(): $data is "235 Authentication successful
"SMTP -> FROM SERVER:235 Authentication successful
CLIENT -> SMTP: MAIL FROM:
SMTP -> get_lines(): $data was ""SMTP -> get_lines(): $str is "421 invalid sender domain '192.168.16.54' (misconfigured dns?) http://postmaster.1and1.com/error-messages/#invalidsenderdomain
"SMTP -> get_lines(): $data is "421 invalid sender domain '192.168.16.54' (misconfigured dns?) http://postmaster.1and1.com/error-messages/#invalidsenderdomain
"SMTP -> FROM SERVER:421 invalid sender domain '192.168.16.54' (misconfigured dns?) http://postmaster.1and1.com/error-messages/#invalidsenderdomain
SMTP -> ERROR: MAIL FROM command failed: 421 invalid sender domain '192.168.16.54' (misconfigured dns?) http://postmaster.1and1.com/error-messages/#invalidsenderdomain