My e-mails end up in spam

Hi,
Many of the e-mails I send to my customers end up in spam. This is usually the first email I send to someone. Or someone writes to me asking for an offer, I write back and it ends up in spam. When we exchange a few messages, it's okay, but often one will end up in spam again. As a result, I lost many interested parties who noticed my meil in spam after a month and reported the problem to me. Unfortunately, not seeing my answers, they decided to work with someone else.

I have mail on OVH - Roundcube, but connected to Gmail. Im writing emails from Gmail.

Another problem is that messages appear on Roudcube and after around 3 hours they are redirected to Gmail and appear on the Gmail. It is too long!

Does anyone have any idea what is wrong here?
I will be grateful for help.