Unfortunately there is a known issue with Outlook 2016 and Outlook 2019 where Horizontal lines might show at random in your email:

https://mosaico.io/email-client-tricks/outlook-2016-weird-1px-horizontal-lines/ 

There's no real fix to the problem right now apart from trying to make small changes to font-size and line-height. We are monitoring the issue to see if a solution arises but ultimately it needs to be fixed by Outlook at some point.

In knak we make use of custom webfonts to help our clients get as close to their brand guidelines as possible in their email content. These custom webfonts always have a fallback "web safe" font that is similar in style to the custom font. This is because not all email clients will be able to render the custom web font and will show the fallback instead.

Custom web fonts aren't supported everywhere but here are the email clients where they are supported:

When an email is being forwarded, some email clients make changes to the code of the forwarded message. This can include stripping out certain HTML elements, wrapping your email in a blockquote, or inserting additional classes to your code—all of which can break your design and make the email less functional for the recipient of the forwarded email. 

Knak emails utilize the Forward to Friend token {{system.forwardToFriendLink:default=edit me}} . This will insert the necessary forward link for your emails. This prevents Marketo from adding an ugly line of copy at the very bottom of your emails and will optimize your emails when being forwarded.
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Knak builder makes use of a coding technique called VML buttons in order to ensure your buttons look great across all the email clients, including Outlook. This technique essentially creates two versions of the button. The first uses a conditional comment to target Outlook desktop clients and uses VML markup in Outlook specific code to style the button (round corners). 

Some customers might not be able to use this VML version of the button due to using an ESP that doesn't support VML tracking (Eloqua) or due to wanting to avoid the issues with button code manipulation.

If you're not seeing rounded corners on your buttons in Outlook 07/10/11/13 it is most likely due to one of these scenarios. Feel free to reach out to us using the chat to confirm this. In these cases the corners of the buttons will always be square and only the link of the email itself will be clickable.
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You are able to upload animated image such as .gif in knak. However some email clients do not support the animation and will display only the first frame of the GIF. You should ensure the first frame is relevant to the content without the animation.
Here is an interesting articles with more details: https://litmus.com/blog/a-guide-to-animated-gifs-in-email

For information on adding a video click here: https://help.knak.io/en/articles/3292868-adding-a-video-to-your-email
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Yes, you have the ability to change the padding and spacing on mobile devices by clicking on the desktop and mobile icon.

In order to render correctly on mobile, the content of each column should be under the same row (image, text, button). In order for the buttons to align, the paragraph length should ideally be the same in all 3 columns and you can use line spacing and play with padding to align them.

Long links and text should be avoided as Outlook does not support them. They will cause rendering issues as the text will not wrap properly.

For links, a button with a link or a text link with shorter 'Text to display' would resolve this issue. A non-breaking space could be added to a longer text to help render the email correctly.

If you are experiencing a plain text link being autolinked after you send out your email, this is because some mail clients autolink (and colour) plain text links. We can only control link colours for links added in Knak, and links added in the MAP after export.

For this reason, URLs should always be linked and styled as desired in Knak to avoid any of this undesirable behaviour. It's best practice in emails to always link what you want to be linked to control the styling, and to make sure it gets tracked.

If you want it to look plain text and you want to avoid an autolink added by the mail provider: add it as a hyperlink in the text box, uncheck 'Underline links', set the link colour in the editor the same as your text colour.

If your links are being directed to a broken page and you use global link parameters, check if there is a question mark in between two link parameters. This is likely causing it to break. To fix this they should go to link review and update the '?' to an '&' (e.g. html?node to html&node)

Need more help? Contact support via live chat within Knak using the chat bubble in the bottom right corner or email support@knak.com