Why emails end up in spam and how to explain it?

Question

Email

Help me please!
The situation is this: I work with a foreign company. They set up mail for me on their domain.
Problem: letters come with a note that it was not possible to confirm that the domain belongs to the sender, Yandex says "no signature".
I write upstairs that perhaps there is no DKIM signature, I explain on my fingers what it is. They contact those who set up mail for them.
The answer comes that everything is in order, probably someone marked it as spam.
Bottom line: not a single response was received to the mailing list on the warm base after the exhibition.
Tell me, what else could be the problem?
And how to explain it

Answer 1

SPF check