Python smtplib发送邮件返回250 OK但Gmail收件人未收到（企业域名发件方）技术排查问询

1. Is your Python SMTP usage correct? #

Yep, your code looks totally solid. Here’s why:

• You’re using SMTP_SSL with port 465, which is the standard secure setup for SMTP connections.
• The debug logs confirm every step works: authentication succeeds, sender/recipient are accepted, the message is sent and acknowledged by your relay server.
• It reliably delivers to same-domain colleagues, so your core sending logic is valid.
• The send_message method correctly handles your MIMEText object, and basic headers (Subject, From, To) are properly set.

Small tweak suggestion: You could update the From header to include a friendly name (e.g., msg["From"] = "Your Company <no-reply@your-domain.com>") which might help with Gmail’s filtering, but this isn’t a critical error in your current code.

2. Is this a deliverability/authentication issue rather than a code problem? #

Absolutely. Here’s the breakdown:

• Your SMTP relay (HostGator) accepts the message and confirms it’s sent, so your code is doing its job. The problem is downstream at Gmail’s end.
• Same-domain emails work, which rules out general sending failures. Gmail’s filters are way stricter for external senders, especially when authentication protocols are missing or misconfigured.
• The fact that the email doesn’t show up even in spam suggests Gmail is silently rejecting it—this is a common response to unauthenticated or low-reputation messages. Even if past emails worked, small changes to DNS records, IP reputation, or Gmail’s filter rules could trigger this.

3. Specific checks to request from your infrastructure/ops team #

Since you don’t have cPanel access, ask them to verify these critical items:

• SPF Record: Ensure your domain’s DNS has a valid SPF record that includes your sending SMTP server (gatorxxxx.hostgator.com). For example, a record like v=spf1 include:hostgator.com ~all would authorize HostGator’s servers to send on your domain’s behalf.
• DKIM Configuration: Confirm DKIM is enabled for your domain. They need to check that the private key is installed on the HostGator server, and the corresponding public key is published in your domain’s DNS (as a TXT record with a name like default._domainkey.your-domain.com).
• DMARC Record: Check if a DMARC record exists in your DNS. Even a basic record like v=DMARC1; p=none; rua=mailto:dmarc@your-domain.com will help you receive reports on authentication failures. If it’s missing, adding it can improve deliverability.
• Reverse DNS (PTR): Verify that the sending IP address has a PTR record pointing back to your domain. Gmail requires valid PTR records to trust incoming emails.
• IP Reputation: Ask if the sending IP has any negative marks in Gmail’s internal reputation system (even if it’s not on public blacklists). Recent spam complaints or sudden increases in sending volume could hurt reputation.
• Recent DNS Changes: Check if any updates were made to your domain’s DNS records (like SPF/DKIM) in the last few days that might have broken authentication.

Bonus Quick Test #

If your team can’t immediately check all the above, ask them to send a test email directly from the HostGator server (using tools like sendmail or telnet) to a Gmail address. If that also fails, it confirms the issue is with the server/domain configuration, not your code.

内容的提问来源于stack exchange，提问作者Hamza Khan