What are the benefits of it over a simple link in the email?
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+1
The benefits from my point of view:
- you save one click when you aren't interested to read the email content and you just want to open the issue page directly. Now you need to click on the email subject (to open the email content) and then search visually for the URL to click it.
- the link to the issue page is more clearly highlighted.

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Exactly that. Also, hopefully it's just a start of more useful functions in the next versions – for example, the ability to mark an issue as resolved from within the mailbox.
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This is a great feature but I could not see "View issue" link on GMail (Redmine 3.1.1 + patch).
Is this works fine on your environment? Please let me know how can I make the patch work.
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It won't work right away. You'll need to "register with Google":https://developers.google.com/gmail/markup/registering-with-google.
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"Here":https://www.google.com/webmasters/markup-tester/ you can find a tester for email markups offered by Google.
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> Here you can find a tester for email markups offered by Google.

Please note that this may determine the markups to be invalid if you're using a localhost URL – e.g. when you're testing Redmine from local env.
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The patch works great on Redmine 3.1.2. Below is a notification email received from Redmine after my registration request was approved by Google.

!ViewIssue.png!

It'll be a nice addition to Redmine notifications emails for Gmail/Inbox users.
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This patch works also on Redmine 3.2.0 and 3.2.1.
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Oh, great! I just wanted to do this.
I tried it with Redmine 3.3.0, but without success.
When I look into code (email source), there are so e weird 3D added everywhere:
<pre>
<div itemscope itemtype=3D"http://schema.org/EmailMessage">
</pre>

but IMHO it hsould be like this:
<pre>
<div itemscope itemtype="http://schema.org/EmailMessage">
</pre>

When I removed thode 3Ds, I got better results with validator: https://www.google.com/webmasters/markup-tester/u/0/

.erb template looks good, any clue, where those "3D" string are coming from?
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Some clue... https://www.sitepoint.com/community/t/what-does-3d-mean-in-this-html-email/7033
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Very useful for Gmail users. Users can access an issue quickly, without opening an email message.
Setting target version to 3.4.0.
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So, this feature is for Gmail users only and each instance needs to register at Google before using it?
I'd like to get more feedback about this. Sounds to me like a good plugin idea instead of adding it to the core.
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Me too! I was playing with this idea (which is really nice by the way and there is a lot of ways to implement it), but I ended with the conclusion, that's more oriented to one-domain based hosted products, so the owner of product can ask Google for adding a scheme (like ie Trello).
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We've had "GMail Actions / Schema.org Markup in Planio":https://support.plan.io/news/117 for a few months now. At Planio, it was easy to enable this for everyone, because we send all email from one specific email address @@plan.io@ and we just had to request white-listing of that address with Google to make it work for all customers.

Adding this to Redmine, however, would mean that every Redmine admin would have to get their sender address white-listed by Google individually to use this feature. On the other hand, it does not hurt to have this markup in Redmine. It does not break anything and will just be ignored by Google if the sender isn't white-listed.

Jean-Philippe, if you decide that you would like to add it, I would be happy to prepare a patch for the code we use at Planio, which builds the Schema.org tags in a view helper and could therefore be used not only in issues, but all other notification types as well. This approach also does not "pollute" the mailer views too much with additional markup.
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+1

Large number of people who use their own pop3 or imaps, use gmail as email client... And it would have "wow" effect for new users...

However it does bring some ambiguity in some use-case scenarios, don't forget that Redmine is often used as helpdesk platform where "view issue" is possible, but preffered action is to reply to that email and Redmine parses that reply, instead of having users click "view issue"...

Probably best way to implement this (if it's to be implemented) would be to put in email settings "Enable/disable" email action buttons...
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