使用未加密SMTP（未采用TLS、SSL、MIME）会降低邮件送达率吗？

Great questions—these are critical points anyone setting up email delivery should get right, so let’s break this down clearly.

1. Does using unencrypted SMTP hurt email deliverability? #

Absolutely, and the impact can be significant. Here’s why:

• Major providers block or flag plaintext SMTP: Services like Gmail, Outlook, and Yahoo have strict security policies. They’ll often route plaintext emails straight to spam folders, or even reject the connection entirely if your server tries to send unencrypted mail over standard ports (like 25).
• Reputation hits: Anti-spam systems (think Spamhaus, Google’s Postmaster Tools) factor in whether your emails are encrypted as part of their sender reputation scoring. A plaintext setup signals "untrusted" to these systems, which lowers your overall score and makes it harder for future emails to reach inboxes.
• Port restrictions: Many ISPs and email servers now block plaintext traffic on port 25 entirely to cut down on spam and eavesdropping. If your SMTP is unencrypted, your messages might never even reach the recipient’s server.

2. What about SMTP without TLS/SSL/MIME—does that hurt deliverability? #

First, a quick clarification: MIME isn’t an encryption mechanism—it’s the standard that lets emails support attachments, HTML formatting, and non-text content. Skipping MIME won’t directly hurt deliverability, but it’ll make your emails unreadable (no images, no attachments) which is a terrible user experience.

Now for TLS/SSL:

• TLS is non-negotiable for modern deliverability: Almost every legitimate email server now requires TLS encryption (usually via STARTTLS on port 587; SSL on port 465 is older but still supported by some). If your SMTP doesn’t use TLS, you’ll face the same issues as plaintext SMTP: spam folder routing, connection rejections, and reputation damage.
• SSL is outdated but still better than nothing: While SSL has been replaced by TLS as the secure standard, using SSL is still preferable to sending plaintext. That said, most modern servers prioritize TLS, so focus on configuring STARTTLS if you can.

Quick Best Practices #

• Always use STARTTLS (port 587) for SMTP connections—it’s the current industry standard.
• Make sure your SMTP server has a valid, trusted SSL certificate (self-signed certificates might be rejected by recipient servers).
• Don’t skip MIME—even if it doesn’t affect deliverability, it’s essential for functional, professional emails.

内容的提问来源于stack exchange，提问作者kratos