Why does Phpmailer suddenly stop sending messages?

Question

Email

I use phpmailer via smtp on landingos to send requests. Everything suits, works, BUT! at one fine and usually unexpected moment, it stops sending mail, which is accessed through phpmailer.
It makes no sense to talk about the limits of departures, there are not many applications. Tell me how to insure? Maybe there are more reliable ways ... I would be very grateful!

Answer 1

SERVER -> CLIENT: 550 spam message rejected.

The server considers this message to be spam and therefore refuses to send it.
To avoid this, you need to at least add your server to the trusted list using the SPF record in DNS.
Ideally, also configure DKIM.