{"id":19010913,"date":"2026-09-03T16:43:15","date_gmt":"2026-09-03T16:43:15","guid":{"rendered":"https:\/\/wordpress.org\/support\/topic\/microsoft-graph-bcc-not-working\/"},"modified":"2026-09-03T16:43:15","modified_gmt":"2026-09-03T16:43:15","slug":"microsoft-graph-bcc-not-working","status":"publish","type":"topic","link":"https:\/\/wordpress.org\/support\/topic\/microsoft-graph-bcc-not-working\/","title":{"rendered":"Microsoft Graph Bcc not working"},"content":{"rendered":"\n

FluentSMTP resets PHPMailer to “mail” mode<\/strong>, not SMTP mode, before every send \u2014 see helpers.php:639<\/a> ($phpmailer->isMail()<\/code>), and no code path in the Outlook provider ever calls isSMTP()<\/code> (only Providers\\Smtp\\Handler.php:45<\/code> does that, for the plain-SMTP connection type).<\/p>\n\n\n\n

That mode matters a lot for Bcc.<\/strong> PHPMailer only omits<\/em> the Bcc:<\/code> header from the generated MIME when Mailer === 'smtp'<\/code> (because in real SMTP mode it delivers Bcc via an extra invisible RCPT TO<\/code> command instead). Since Outlook’s mode is 'mail'<\/code>, PHPMailer does<\/strong> write a Bcc:<\/code> header into the raw MIME \u2014 so at the PHP level, Bcc is present, right alongside Cc.<\/p>\n\n\n\n

But the Outlook handler throws that context away.<\/strong> Outlook\/Handler.php:77-102<\/a> (sendViaApi()<\/code>) just grabs $this->phpMailer->getSentMIMEMessage()<\/code>, base64-encodes it, and POSTs the raw MIME blob to https:\/\/graph.microsoft.com\/v1.0\/me\/sendMail<\/code> (Outlook\/API.php:133-142<\/a>) with Content-Type: text\/plain<\/code>. That’s it \u2014 no structured JSON recipients are sent.<\/p>\n\n\n\n

Microsoft Graph’s raw-MIME ingestion at \/me\/sendMail<\/code> parses To:<\/code> and Cc:<\/code> out of the header block fine, but doesn’t route delivery off a Bcc:<\/code> header found in raw MIME content.<\/strong> It only honors Bcc when it’s supplied as a structured bccRecipients<\/code> field. This is a known Graph API quirk with the “upload raw MIME” send path \u2014 not something this plugin’s header-building is getting wrong.<\/p>\n\n\n\n

Proof it’s architectural, not accidental:<\/strong> other providers in this same FluentSMTP install handle Bcc correctly because<\/em> they don’t use the raw-MIME path. BaseHandler::setAttributes()<\/code> (around BaseHandler.php:186-206<\/a>) already extracts getBccAddresses()<\/code> into $attributes['headers']['bcc']<\/code> for every provider \u2014 and Mailgun’s handler actually uses it (Mailgun\/Handler.php:65,152<\/a>), building its own bcc<\/code> field in its API call. The Outlook handler is the one provider that ignores this pre-computed data entirely.<\/p>\n\n\n\n

Ways to actually fix it (not applying any of these yet):<\/strong><\/p>\n\n\n\n

Best fix<\/strong>\u00a0\u2014 change\u00a0Outlook\\Handler::sendViaApi()<\/code>\u00a0to stop using the raw-MIME\u00a0\/sendMail<\/code>\u00a0endpoint, and instead build the structured Graph JSON payload (message.toRecipients<\/code>\u00a0\/\u00a0ccRecipients<\/code>\u00a0\/\u00a0bccRecipients<\/code>), pulling bcc from the\u00a0headers.bcc<\/code>\u00a0array\u00a0BaseHandler<\/code>\u00a0already computes \u2014 mirroring what Mailgun’s handler does today.<\/p>\n\n\n\n

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