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Identify where the send is failing
A message being “sent” can mean different things: your PHP code called the send method, your SMTP server accepted the message, or the message reached the recipient’s inbox. These are not equivalent. PHPMailer is an email library with an SMTP client; changing libraries alone will not resolve a blocked connection, rejected credentials, sender-domain authentication failure, or filtering.
Collect the exact PHPMailer error, a sanitized SMTP debug trace, and any full NDR. Remove passwords, OAuth tokens, client secrets, and personal recipient details before sharing logs. Also identify whether the sender uses PHP’s local mail transport, an independent SMTP provider, Outlook.com, or Microsoft 365. Without those details, the cause cannot be determined reliably.
- “Could not connect to SMTP host” or similar: investigate host, DNS, network access, port, and TLS mode.
- Connection succeeds but login fails: verify the account and supported authentication method.
- A Microsoft NDR reports a rejection: use its exact SMTP code to guide the fix.
- PHPMailer reports success but the message is missing: look for filtering, Junk placement, or sender reputation and authentication problems.
If PHPMailer cannot connect to the SMTP server
Check the SMTP hostname against the sending provider’s current instructions, then verify that DNS resolves it and that your hosting provider or firewall allows outbound SMTP traffic. A local DNS problem or blocked connection can look like a PHPMailer defect even though PHPMailer is not the cause. Check the selected port and encryption mode together; do not assume every provider accepts the same configuration.
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PHPMailer’s Microsoft example documents STARTTLS on port 587 and implicit TLS on port 465 as transport examples. These are configuration references, not confirmation that every Microsoft account or service permits those settings. Follow the instructions for the actual sending service and account type.
If SMTP authentication fails
Confirm the username and the sign-in method supported by the sending provider. PHPMailer supports XOAUTH2, but successful use still depends on valid credentials, any required consent, provider configuration, and the account’s policies. A username and password that work in a mail app are not proof that the same authentication method is allowed for an SMTP connection from your PHP application.
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Distinguish consumer Outlook.com from Microsoft 365 and from another SMTP provider before changing settings. Microsoft 365 application sending has its own setup choices and identity or domain requirements; use the instructions for the specific application and environment rather than copying settings intended for a different service.
If Microsoft rejects the message with 550 5.7.515
This code is not a generic PHPMailer syntax error. Microsoft says it indicates that the domain in the message’s 5322.From address did not meet the required authentication level for high-volume senders to Microsoft consumer email services. Microsoft defines high volume here as 5,000 or more messages to those services using the same domain in the 5322.From address; the publication year for the cited Microsoft page is not stated. See Microsoft’s explanation of error 550 5.7.515.
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For this rejection, check the sending domain’s SPF and DKIM results, confirm that a DMARC record exists, and verify alignment: the authenticated identifier must align with the domain in the visible 5322.From address for DMARC to pass. Do not treat a successful PHP send call as evidence that these checks passed; use message headers, DNS records, and the NDR to establish the outcome.
If the message is accepted but does not reach the inbox
Check the recipient’s Junk folder and any mailbox rules or filters first. Then review the sending IP’s reputation, the sender identity, DNS configuration, and domain authentication. Microsoft lists low IP spam reputation as one possible reason for delivery difficulty to Outlook.com or Hotmail recipients, not as a diagnosis of any particular message. Its Outlook.com sender support guidance covers sender-side considerations.
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Inbox placement is not guaranteed by changing one DNS record or SMTP setting. If the sending server accepted the message but there is no NDR and no inbox copy, use the sending provider’s logs or message trace where available to determine what happened after handoff.
Quick Recap
PHPMailer and Microsoft references
- PHPMailer troubleshooting guide for common connection and configuration problems.
- PHPMailer SMTP example for a code-level transport configuration reference; confirm current settings with your provider.
- PHPMailer project documentation for SMTP and XOAUTH2 capabilities.
- Microsoft 365 application sending guidance for Microsoft 365-specific setup choices.
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