When sending an email to another server, the following error may be present in the Exim log:
2021-01-04 22:36:59 1kwd9W-0004fc-RL == firstname.lastname@example.org R=dkim_lookuphost T=dkim_remote_smtp defer (-18) H=remotehostdomain.tld [192.0.2.0]: Remote host closed connection in response to initial connection
The above error is due to the remote host or server closing the server's connection trying to send a message.
You will need to contact the server administrator of the server that you are trying to send an email. They will need to review their server's logs to provide you with more information.