Exactly the same problem here - although I have some old messages which are displayed correctly.
A possible solution, on our side now everything work flawlessly.
Change line 1878 in MailController.class.php
from:
"date" => $msg->getReceivedDate() instanceof DateTimeValue ? ($msg->getReceivedDate()->isToday() ? format_time($msg->getReceivedDate()) : format_datetime($msg->getReceivedDate())) : lang('n/a'),
to:
"date" => $msg->getSentDate() instanceof DateTimeValue ? ($msg->getSentDate()->isToday() ? format_time($msg->getSentDate()) : format_datetime($msg->getSentDate())) : lang('n/a'),
and line 163 in \application\views\mail\view.php
from:
$info_date = $info->getReceivedDate() instanceof DateTimeValue ? ($info->getReceivedDate()->isToday() ? format_time($info->getReceivedDate()) : format_datetime($info->getReceivedDate())) : lang('n/a');
to:
$info_date = $info->getSentDate() instanceof DateTimeValue ? ($info->getSentDate()->isToday() ? format_time($info->getSentDate()) : format_datetime($info->getSentDate())) : lang('n/a');
Greetings
Stefan