Emails sent from nextcloud contain an empty Cc: field

ehrt74 · February 11, 2026, 8:14pm

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Nextcloud Server version (e.g., 29.x.x):
- hosted by ionos
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Summary of the issue you are facing:

emails sent using the nextcloud mail app have an empty Cc: field (if no Cc: is selected). These are then rejected by simplelogin.io

Steps to replicate it (hint: details matter!):

create an account at simplelogin.io and register the email address you use in nextcloud as your email address at simplelogin
from another account send an email to your alias at simplelogin.io. the email will be forwarded to the nextcloud account
reply to the email from within the nextcloud email app without setting a Cc:. the reply will be rejected by simplelogin.io because the Cc: field is present but empty.

Log entries

Nextcloud

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hosted by ionos

Web Browser

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cromite 144

Web server / Reverse Proxy

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hosted by ionos

Configuration

Nextcloud

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hosted by ionos

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jtr · February 13, 2026, 4:14pm

Yes, it appears some headers with empty values (such as Cc: are currently included in sent messages (which is generally okay).

Does adding the following to fix your problem?

// Remove headers where the value is empty, null, or just whitespace
$headers = array_filter($headers, function($value) {
    return !empty($value) && (is_string($value) ? trim($value) !== '' : true);
});

It needs to go right here (line number is 129 here but may be different in your Mail app version; the point is it needs to go just above the addHeaders() call.

github.com/nextcloud/mail

lib/Service/MailTransmission.php

4570c30ed


      
          		// but we already set the 'references' header for an email so we could support multiple references
          		// Get the previous message and then concatenate all its "References" message ids with this one
          		if (($inReplyTo = $localMessage->getInReplyToMessageId()) !== null) {
          			$headers['References'] = $inReplyTo;
          			$headers['In-Reply-To'] = $inReplyTo;
          		}
          
          		if ($localMessage->getRequestMdn()) {
          			$headers[Horde_Mime_Mdn::MDN_HEADER] = $from->toHorde();
          		}
          
          		$mail = new Horde_Mime_Mail();
          		$mail->addHeaders($headers);
          
          		$mimeMessage = new MimeMessage(
          			new DataUriParser()
          		);
          		$mimePart = $mimeMessage->build(
          			$localMessage->getBodyPlain(),
          			$localMessage->getBodyHtml(),
          			$attachmentParts,

EDIT: PR pending to address this: fix(MailTransmission): Handle clients/servers picky about empty headers by joshtrichards · Pull Request #12420 · nextcloud/mail · GitHub

ehrt74 · February 13, 2026, 6:30pm

Many thanks for your work I’m afraid I’m on a managed NextCloud instance with no access to the code

jtr · February 13, 2026, 9:24pm

Fair enough. Turns out my quick attempt needs some refinement anyhow. The PR is active so hopefully will be in an upcoming release and will address situation.