I've been working on Quail for a while, often dealing with emails and such, and I've come across some pretty interesting stuff I'd like to share with everyone.

A Lot of People Misspell Their Own Email Addresses

Common mistakes include:

  • Writing .com as .con, .cmo, .cm, .comm
  • Writing gmail as gmial, gmal, gmmail, gmil, gnail
  • Writing qq.com as q.com, qq.cn

All of these mistakes obviously result in emails not being received. Because once these errors are identified, attempts to send emails cease, naturally resulting in no emails received.

There's also the issue of people misspelling the prefix of their email (the part before the @). Some of these cases can be detected as well, also leading to emails not being received.

Recipient Emails Also Have Scores

Many email sending services offer Email Validation services. For example, Quail uses Sendgrid and Mailgun. Taking Sendgrid as an example, providing an email address can yield the following judgments:

  1. Whether this address can receive emails: Yes, Risky, Do Not Send
  2. Score: When an address is identified as "Risky", the lower the score, the higher the risk. If the risk is high, then the email might not be sent.
  3. Detailed reasons, including but not limited to the following:
    1. No MX records configured
    2. It’s a disposable address (mentioned below)
    3. This email address has previously rejected emails
    4. The domain is uncommon
    5. The domain is too new
    6. A lot of spam has been sent from this domain
    7. The top-level domain is risky (mentioned below)
    8. The email address does not exist (common in cases of misspelling)

So, it's not just senders who get scored; recipients do too.

Using Special Domains Can Lower Email Credibility

Many trendy new domains are now available, such as .xyz, .cf, .cloud, etc.

Using these domains for your email can carry certain risks.

Some domains are considered high-risk overall. If the email configuration is not set up properly, or if the corresponding website is not well-managed, or if TLS is not implemented, then some email validation services might consider it high-risk, leading to email services refusing to send mails.

Because email services (like Quail) aim to maintain their sending service's Reputation, they might directly refuse to send emails to addresses considered high-risk.

Using Temporary Emails

There are many temporary email and temporary mobile number services available online for receiving SMS messages. Using them for registration might also result in not receiving emails or messages.

These can also be detected. Again, to maintain their sending service's Reputation, services will not send emails to these addresses (Quail included) because it would be pointless.

Email Relay Services Are Considered Low to Medium Risk

Typical examples include kill-the-newsletter.com, readwise.io, omnivore.app, ino.to, and Apple's privaterelay.appleid.com. It's not that they're bad, but they are indeed considered medium to low risk by the aforementioned Validation, lowering their score.

If a sending service has its own risk control strategy, due to the higher risk level of these services, emails might not be received.

A Lot of People's Inboxes Are Full

It sounds unbelievable, but in this day and age, email inboxes can actually be full. From what I've seen in sending emails, some people's inboxes are already full... Naturally, they won't receive any emails then.

Mysterious Network Issues from Ancient Eastern Countries

Common with Sina, QQ, NetEase emails. Sometimes they mysteriously reject emails, with observed reasons including:

  1. Timeouts: In this case, emails are not received at all.
  2. Emails sent successfully but not appearing in the inbox: This often occurs when the email content contains something these ancient countries do not like.

That's all.

Everything mentioned here is from the perspective of the reader. For those providing email services, like Quail, there are even more issues to be aware of. If you're also in the business of sending emails and encounter some tough problems that are hard to solve, feel free to ask me; maybe I can help.