TYPO3 Core

To test, simply send a mail using the mail api where the 76th chacacter is a "dot":

/** @var MailMessage $mail */
$mail = GeneralUtility::makeInstance(MailMessage::class);
$mail->to(new Address('info@domain.tld'));
$mail->from(new Address('info@domain.tld'));
$mail->subject('Test');
$mail->html('xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx12345.xxx');
$mail->send();

In the resulting mail, the "dot" has been replaced by a double dot:

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx12345..xxx

This also occures, when the first letter of a line is a "dot":

$mail->html('.xxx');

This results in:

..xxx

In most environments this will be noticed only accidentally (if at all), but it happens. And especially when using the mail api intensively (e.g. for sending newsletters to lots of recipients), this can cause unpleasant results (dead links or invalid css classes).

It shouldn't be an integrators or developers part so set the sendmail option "-i" manually per instance (most won't do because they don't know why). It should be default.

It seems that TYPO3 retrieves the default sendmail command from PHP (php.ini). In my case (a fresh 10 LTS installation on mittwald) it is set like "sendmail_path = /usr/sbin/sendmail -t"). Should the core automatically extend it with "-i" if missing, despite? Or is this only a "mittwald" issue?

I can't remember having had this problem on any other hosting than Mittwald. Maybe a check in the Reports module would be enough if sendmail is the configured transport.
When testing a clients bug report locally for "wrong urls with double dots" and you use something like ddev with mailhog you will never find the source of the problem :/