Hello @Cres.Cor ,
Here are some general best practices for delivery.
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
If you're wanting more in-depth, specialized insight on your current deliverability, it'd be worth speaking with one of our Delivery agents.
Regarding the subject line personalization, please note that subject line personalization doesn't display when previewing an email or when sending a test email. To see what the personalization looks like, you could try copying your email and sending yourself the copy as a live send (i.e. your email as a contact in a list on its own).
I'd recommend checking out our main article on subject line personalization for more details, and for common workarounds / quirks to look out for.
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