Hello @NewLifenl ,
If this continues to happen sporadically, there's likely something on their network or IT system that's blocking the emails from going through for whatever reason. If they're using Gmail, have they checked their Promotions folder?
Deliverability can have a lot of variables, from elements on our side, on your side, on the recipient's side, and in their email system company's side. It's also a generally good idea to set up additional authentication for your emails - whether that's through us if you have a free domain (e.g. Gmail, Yahoo, AOL, etc.) or via self-authentication if you have your own domain (such as through a business or other organization).
If you'd like to learn more about safelisting, and what it entails:
Safelisting domains in a security system
Safelisting email addresses in an email client or security system
Here are some general best practices for deliverability.
If you're wanting more in-depth, specialized insight on your current deliverability, it'd be worth speaking with one of our Deliverability agents.
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