Handle message size limits on SMTP

Steps to reproduce

0. Have a server with a message size restriction, e.g. 10MB
1. Send email with a 11.5MB attachment

Expected behavior

Message is sent

Actual behavior

https://nextcloud-gmbh.sentry.io/issues/4032642026

Horde_Smtp_Exception: Message size exceeds fixed limit
#14 /mail/vendor/nextcloud/horde-smtp/lib/Horde/Smtp.php(1076): Horde_Smtp::_getResponse
#13 /mail/vendor/nextcloud/horde-smtp/lib/Horde/Smtp.php(659): Horde_Smtp::send
#12 /mail/vendor/bytestream/horde-mail/lib/Horde/Mail/Transport/Smtphorde.php(143): Horde_Mail_Transport_Smtphorde::send
#11 /mail/vendor/bytestream/horde-mime/lib/Horde/Mime/Part.php(1620): Horde_Mime_Part::send
#10 /mail/vendor/bytestream/horde-mime/lib/Horde/Mime/Mail.php(479): Horde_Mime_Mail::send
#9 /mail/lib/Service/MailTransmission.php(272): OCA\Mail\Service\MailTransmission::sendMessage
#8 /mail/lib/Service/MailTransmission.php(344): OCA\Mail\Service\MailTransmission::sendLocalMessage
#7 /mail/lib/Service/OutboxService.php(142): OCA\Mail\Service\OutboxService::sendMessage
#6 /mail/lib/Controller/OutboxController.php(227): OCA\Mail\Controller\OutboxController::send
#5 /var/www/cloud.domain.com/nextcloud/lib/private/AppFramework/Http/Dispatcher.php(230): OC\AppFramework\Http\Dispatcher::executeController
#4 /var/www/cloud.domain.com/nextcloud/lib/private/AppFramework/Http/Dispatcher.php(137): OC\AppFramework\Http\Dispatcher::dispatch
#3 /var/www/cloud.domain.com/nextcloud/lib/private/AppFramework/App.php(183): OC\AppFramework\App::main
#2 /var/www/cloud.domain.com/nextcloud/lib/private/Route/Router.php(315): OC\Route\Router::match
#1 /var/www/cloud.domain.com/nextcloud/lib/base.php(1052): OC::handleRequest
#0 /var/www/cloud.domain.com/nextcloud/index.php(36): null

Horde_Mail_Exception: Message size exceeds fixed limit
#12 /mail/vendor/bytestream/horde-mail/lib/Horde/Mail/Transport/Smtphorde.php(145): Horde_Mail_Transport_Smtphorde::send
#11 /mail/vendor/bytestream/horde-mime/lib/Horde/Mime/Part.php(1620): Horde_Mime_Part::send
#10 /mail/vendor/bytestream/horde-mime/lib/Horde/Mime/Mail.php(479): Horde_Mime_Mail::send
#9 /mail/lib/Service/MailTransmission.php(272): OCA\Mail\Service\MailTransmission::sendMessage
#8 /mail/lib/Service/MailTransmission.php(344): OCA\Mail\Service\MailTransmission::sendLocalMessage
#7 /mail/lib/Service/OutboxService.php(142): OCA\Mail\Service\OutboxService::sendMessage
#6 /mail/lib/Controller/OutboxController.php(227): OCA\Mail\Controller\OutboxController::send
#5 /var/www/cloud.domain.com/nextcloud/lib/private/AppFramework/Http/Dispatcher.php(230): OC\AppFramework\Http\Dispatcher::executeController
#4 /var/www/cloud.domain.com/nextcloud/lib/private/AppFramework/Http/Dispatcher.php(137): OC\AppFramework\Http\Dispatcher::dispatch
#3 /var/www/cloud.domain.com/nextcloud/lib/private/AppFramework/App.php(183): OC\AppFramework\App::main
#2 /var/www/cloud.domain.com/nextcloud/lib/private/Route/Router.php(315): OC\Route\Router::match
#1 /var/www/cloud.domain.com/nextcloud/lib/base.php(1052): OC::handleRequest
#0 /var/www/cloud.domain.com/nextcloud/index.php(36): null

Horde_Mime_Exception: Message size exceeds fixed limit
#11 /mail/vendor/bytestream/horde-mime/lib/Horde/Mime/Part.php(1642): Horde_Mime_Part::send
#10 /mail/vendor/bytestream/horde-mime/lib/Horde/Mime/Mail.php(479): Horde_Mime_Mail::send
#9 /mail/lib/Service/MailTransmission.php(272): OCA\Mail\Service\MailTransmission::sendMessage
#8 /mail/lib/Service/MailTransmission.php(344): OCA\Mail\Service\MailTransmission::sendLocalMessage
#7 /mail/lib/Service/OutboxService.php(142): OCA\Mail\Service\OutboxService::sendMessage
#6 /mail/lib/Controller/OutboxController.php(227): OCA\Mail\Controller\OutboxController::send
#5 /var/www/cloud.domain.com/nextcloud/lib/private/AppFramework/Http/Dispatcher.php(230): OC\AppFramework\Http\Dispatcher::executeController
#4 /var/www/cloud.domain.com/nextcloud/lib/private/AppFramework/Http/Dispatcher.php(137): OC\AppFramework\Http\Dispatcher::dispatch
#3 /var/www/cloud.domain.com/nextcloud/lib/private/AppFramework/App.php(183): OC\AppFramework\App::main
#2 /var/www/cloud.domain.com/nextcloud/lib/private/Route/Router.php(315): OC\Route\Router::match
#1 /var/www/cloud.domain.com/nextcloud/lib/base.php(1052): OC::handleRequest
#0 /var/www/cloud.domain.com/nextcloud/index.php(36): null

OCA\Mail\Exception\ServiceException: Could not send message: Message size exceeds fixed limit
#9 /mail/lib/Service/MailTransmission.php(274): OCA\Mail\Service\MailTransmission::sendMessage
#8 /mail/lib/Service/MailTransmission.php(344): OCA\Mail\Service\MailTransmission::sendLocalMessage
#7 /mail/lib/Service/OutboxService.php(142): OCA\Mail\Service\OutboxService::sendMessage
#6 /mail/lib/Controller/OutboxController.php(227): OCA\Mail\Controller\OutboxController::send
#5 /var/www/cloud.domain.com/nextcloud/lib/private/AppFramework/Http/Dispatcher.php(230): OC\AppFramework\Http\Dispatcher::executeController
#4 /var/www/cloud.domain.com/nextcloud/lib/private/AppFramework/Http/Dispatcher.php(137): OC\AppFramework\Http\Dispatcher::dispatch
#3 /var/www/cloud.domain.com/nextcloud/lib/private/AppFramework/App.php(183): OC\AppFramework\App::main
#2 /var/www/cloud.domain.com/nextcloud/lib/private/Route/Router.php(315): OC\Route\Router::match
#1 /var/www/cloud.domain.com/nextcloud/lib/base.php(1052): OC::handleRequest
#0 /var/www/cloud.domain.com/nextcloud/index.php(36): null

Mail app version

3.0.0

Mailserver or service

No response

Operating system

No response

PHP engine version

None

Web server

None

Database

None

Additional info

No response

Start in lib/Service/MailTransmission.php, following the send flow through OutboxService and OutboxController; the reported stack trace identifies these entry points and the Horde SMTP and MIME layers involved. Reproduce the 10 MB server limit with an oversized attachment, then determine the intended handling for the size exception and verify that sending produces the appropriate result.