We have developed a feature that creates reservations. The client receives an email containing a link which is clickable only once.

Some of those reservations fail and I can't figure out why. I think that some mail clients (incl. webbased) check the links inside a mail before exposing the mail to the reader. Maybe due to malware scanning, tracking, status code checks,..

Question: Do you know any mail client which behaves like that?

Bonus question: Do those clients stop when the url is defined in robots.txt ?

Edit: I'm currently checking access.log for those links and try to find different access to the reservation links. Hour log files are HUGE and this takes like forever on several servers..

1 Answer 1


I have to admit that I have not followed e-mail clients much over the years. However, I do not think that too many if any e-mail clients screen links, however any anti-spam and anti-virus software will. Generally, these e-mails are returned as an attachment in a text e-mail warning that there is a problem. Some anti-spam software will drop the e-mail entirely as an option. It may also be possible that a link in an e-mail is converted to text only but that should not be a big problem.

I am very sure that robots.txt is not a problem either. Robots.txt are for spiders so web browsers should not check robots.txt file as well as any e-mail client.

The first thing I would check is to make sure that your domain name and IP address are not on a blacklist. My favorite is http://mxtoolbox.com/blacklists.aspx. If your site is not on a blacklist, then I would try and find someone who is having trouble and have them forward the e-mail to you. From there, you can check the raw e-mail code for issues.

Another consideration is that some firewalls have anti-spam and anti-virus checking which may contain checks for phishing and certificates. It may be that firewalls are blocking access. In this case, the phishing checks often use blacklists so you may/should know this using the site above. If your links use HTTPS and you do not have a certificate, then I would either change the HTTPS to HTTP or get a certificate and check that someone can access your site using HTTPS.

Otherwise, I am out of ideas. If I think of anything else, I will update this answer.

  • Thank you for your response. I maybe was not clear enough with my question; the mails get delivered 100% and we are not blacklisted, even if the mail gets sent as text, it's about the link, it's only callable one time, sometimes people open the mail click the link and they get a message like "sorry, the link has already been clicked".. just like their mail client/server/anti-spam kinda "pre-checked" that link.
    – Daniel W.
    Apr 16, 2014 at 7:31
  • @DanFromGermany Okay. Thank you for clarifying. Now I understand. I was sure that your e-mail was delivered okay, but somehow modified by some protection mechanism along the way. I have never heard of this behavior before. I would look for some commonality between all the users, e-mail client, anti-spam/virus, firewall, ISP, something. I am out of ideas right now, but I will keep thinking on this for you.
    – closetnoc
    Apr 16, 2014 at 14:18
  • Don't mind! I wil investigate the log files and check wether I see a call to a link that doesn't match the user but maybe a spam filter or something.
    – Daniel W.
    Apr 16, 2014 at 14:25

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