After Server Migration and Domain Change Email Issues

Dear WordPress Community,

I moved an almost ready to launch WordPress Site from one server to another and changed the domain (WordPress 5.0.4).

After migration I got several problems with sending emails by wordpress core.

I´m able to send mails to my @gmail.com and @web.de E-Mail Adress for example but I´m not able to send mails to my business e-mail adress which has absolutely no spam filter. The business mail adresses are hostet by IONOS.

I tried to check mail sending by writing an php file and I uploaded it to my webspace but I didn´t get any errors. All the messages have been accepted.

<?PHP
$sender = ‘test1@domain1.com’;
$recipient = ‘test2@domain2.de’;

$subject = “php mail test”;
$message = “php test message”;
$headers = ‘From:’ . $sender;

if (mail($recipient, $subject, $message, $headers))
{
echo “Message accepted”;
}
else
{
echo “Error: Message not accepted”;
}
?>

My problem is: I checked sending e-mails also from the source server from which I migrated my WordPress instance to the new serverand from the old source server everything works fine. From the source server I´m able to send e-mails to my business e-mail adress….

I need help

Thank you
regards patde22