Email Emails that are not always sent

I use Centmin Mod in Linode with 3 websites currently. The fact is that I've problems with emails, both in WordPress and in Xenforo, because many of them don't appear to be sent, or at least they are not received. This prevents me from creating contact forms or confirming the registration of users in Xenforo via email, since I don't know if they will be received.

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I also receive daily emails sent by the server (WP-CLI) with the title "Undelivered Mail Returned to Sender", which I suppose will come because the same problem.

Any way to solve this? Thank you.