Add info that missspelling email domain name might result in Bad Request error

Added the following in Troubleshooting section
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Why is 400 Bad Request error encountered?
Misspelling of email domain name in the email address might result in 400 Bad Request error as Mailchimp might reject such email, for example, "joe@gmil.com" because Gmail was misspelled.

Author: tanjinhong72

src/connections/destinations/catalog/mailchimp/index.md

@@ -133,6 +133,9 @@ Again, this will **NOT** work for new users. New users will always have their su
 ### Why are my calls with trait arrays not showing up in Mailchimp?
 Mailchimp doesn't support arrays as traits values. This can cause calls to not show up.
 
+### Why is 400 Bad Request error encountered?
+Misspelling of email domain name in the email address might result in 400 Bad Request error as Mailchimp might reject such email, for example, "joe@gmil.com" because Gmail was misspelled.
+
 ## Engage
 
 You can send computed traits and audiences generated using [Engage](/docs/engage/) to Mailchimp as a **user property**. To learn more about Engage, schedule a [demo](https://segment.com/demo/){:target="_blank"}.