Get from server the host name to put it as sender host.

[tompsonx]

Some servers reject messages when the sender host is set to localhost.

[derickr]

It's already possible to change the senderHost, by setting the senderHost property:

Mail/src/transports/smtp/smtp_transport.php

Line 113 in 2741a9a

* @property string $senderHost

— I am not in favour of changing the default, and hence will be closing this PR.

[tompsonx]

But it seems that the senderHost property isn't used anywhere in the code.