Email has gone into one client's junk folder

Don't panic! This happens to the best of us. If your emails usually go into that contact's inbox and this one time they've gone into their spam folder just this once it could just be a temporary reason. It could be that something with the content of that particular email causing it to be placed in the spam folder. Have a look at this article which gives you tips on what to do to end up in the inbox. 

If it's a one-time occurrence and just with this contact it might not be worth worrying about too much. There are so many factors that affect deliverability that it's not possible to guarantee 100% deliverability. 

Solution 

You could ask your contact to move your email to their inbox and ask them to click on add to safe sender by clicking on the following box in outlook:

They will be given the following options. Ask them to click on Add Sender to Safe Senders List. This effectively adds your email address to a local Allow List on their machine. Your emails should always end up in their inbox from now on.

Email has gone into multiple clients' junk folders

If your emails are ending up in the junk folders of many contacts or those from the same company it indicates that there could be a wider problem.

If your emails are usually fine and you don't experience any issues it could be this particular email that is causing problems.  Have a look at this article which outlines things you could to end up in the inbox.

Have a look at the bounce report for this particular email. Are there higher than normal soft bounces? The bounce reasons sometimes give us an idea of what could have caused it. You can then change the content of the email in future. 

Solution 

Do you have a Custom Return Path? If you don't, this could be one reason why your emails might be landing in junk. The custom return path can really help with your deliverability. Reach out to your Account manager to get it setup.