Scheduled send fails in Mail 5.8.0: `MimeMessage::build()` Argument #3 must be bool, array given

Steps to reproduce

Scheduled send fails in Mail 5.8.0: MimeMessage::build() Argument #3 must be bool, array given

Summary

Scheduled sending does not work in Nextcloud Mail 5.8.0.

The global Nextcloud cron works correctly. The Mail OutboxWorkerJob is registered, runs normally, and executes after the scheduled send time. However, the scheduled message is not sent because OutboxWorkerJob fails with a TypeError in MimeMessage::build().

Immediate/manual sending from the Mail UI works.

Environment

• Nextcloud AIO
• Nextcloud AIO version: 13.0.4
• Nextcloud version: 33.0.3
• Mail app version: 5.8.0
• Background jobs mode: Cron
• User timezone: Europe/Bratislava
• Server/container time: correct
• No Mail app update is currently available through OCC/App Store

What I tested

The test message was scheduled for:

21:55 CEST

UTC equivalent:

19:55 UTC

The Mail outbox background job ran after the scheduled time:

OCA\Mail\BackgroundJob\OutboxWorkerJob
Last executed: 2026-05-19T19:57:34+00:00

So the job clearly ran after the message should have been sent.

The message was a plain text test email without attachments.

Mail background job exists and runs

| 82495485209419777 | OCA\Mail\BackgroundJob\OutboxWorkerJob | 2026-05-19T19:57:34+00:00 | null |

Manual execution of the job also works:

sudo docker exec --user www-data -it nextcloud-aio-nextcloud php occ background-job:execute --force-execute 82495485209419777

Output:

Job class:            OCA\Mail\BackgroundJob\OutboxWorkerJob
Arguments:            null
Type:                 timed

Forcing execution of the job

Job executed!

Timezone checks

Host local time:

Tue May 19 09:49:21 PM CEST 2026

Host UTC time:

Tue May 19 07:49:21 PM UTC 2026

User timezone:

Europe/Bratislava

Nextcloud system default timezone:

Europe/Bratislava

The OutboxWorkerJob timestamp is in UTC and matches the expected CEST offset. This does not look like a timezone or cron scheduling issue.

Error from nextcloud.log

At the time when the scheduled message should have been processed, this error appears:

Could not send outbox message 101: OCA\Mail\Service\MimeMessage::build(): Argument #3 ($isPgpEncrypted) must be of type bool, array given, called in /var/www/html/custom_apps/mail/lib/Service/MailTransmission.php on line 138

Relevant part of the trace:

OCA\Mail\Service\MimeMessage->build(
  "Skusobny email\n--\u00a0\u00a0\nTony Kraj\u010d\u00edk\n+421 949 731110\n\u00a0",
  null,
  [],
  false
)

The third argument passed to MimeMessage::build() is an array [], but the method expects a boolean $isPgpEncrypted.

Full relevant log entry

{
  "level": 2,
  "time": "2026-05-19T19:57:34+00:00",
  "app": "mail",
  "scriptName": "occ",
  "message": "Could not send outbox message 101: OCA\\Mail\\Service\\MimeMessage::build(): Argument #3 ($isPgpEncrypted) must be of type bool, array given, called in /var/www/html/custom_apps/mail/lib/Service/MailTransmission.php on line 138",
  "version": "33.0.3.2",
  "occ_command": [
    "occ",
    "background-job:worker"
  ],
  "exception": {
    "Exception": "TypeError",
    "Message": "OCA\\Mail\\Service\\MimeMessage::build(): Argument #3 ($isPgpEncrypted) must be of type bool, array given, called in /var/www/html/custom_apps/mail/lib/Service/MailTransmission.php on line 138",
    "File": "/var/www/html/custom_apps/mail/lib/Service/MimeMessage.php",
    "Line": 35
  }
}

Available Mail OCC commands

There does not seem to be a dedicated OCC command to repair or process the Mail outbox / scheduled-send queue.

mail:account:create
mail:account:debug
mail:account:delete
mail:account:diagnose
mail:account:export
mail:account:export-threads
mail:account:sync
mail:account:train
mail:account:update
mail:clean-up
mail:repair:system-autoresponders
mail:repair:tags
mail:tags:migration-jobs
mail:thread

Expected behavior

The scheduled email should be sent by OCA\Mail\BackgroundJob\OutboxWorkerJob after the scheduled time.

Actual behavior

The scheduled email is not sent.

OutboxWorkerJob runs, but fails with:

MimeMessage::build(): Argument #3 ($isPgpEncrypted) must be of type bool, array given

Conclusion

This looks like a bug/regression in the scheduled send / outbox processing path in Nextcloud Mail 5.8.0.

The job runs correctly, but MailTransmission.php appears to call MimeMessage::build() with arguments in the wrong order or with the wrong type.

Immediate sending through the UI works, but scheduled sending through OutboxWorkerJob fails.

Expected behavior

reported above

Actual behavior

reported above

Mail app version

5.8.0

Nextcloud version

33.0.3

Mailserver or service

No response

Operating system

Nextcloud AIO - Ububtu server 24.0.4

PHP engine version

None

Nextcloud memory caching

No response

Web server

None

Database

None

Additional info

No response