Every writer has to deal with rejections, no matter how seasoned they are. But in order to be rejected, your MS must actually make it through the door. Yet, the number of people who don't read a publisher's guidelines for submitting is still rather high. This is often an overlooked cause of why rejection levels may feel higher than they should be. If a larger publisher can afford the staff to keep submission windows (SW) open all year round, it's great, but a small press may not be able to do the same.
Why is a publisher's submissions page so important? I hear you cry. Let me explain.
1. Submissions are your first contact with a potential future business partner ( as a matter of fact it's more than just a business relationship, but that's beside the point), and first impressions do matter. By following the guidelines, you are essentially saying two things: one is that you respect the time taken to create and update a submission page, and two you are demonstrating skills such as commitment to, and understanding of a potential working relationship.
2. Open submission windows are there for a reason. Technically, it doesn't matter how big a press is, but especially if you are submitting to a small press, time will be the biggest constraint, followed by budget allocation. A window is set after careful consideration is given to the overall schedule (and not just for the current year) and availability of human resources (MS need to be read while the press continues to run). Take advantage of these SW. There are many sites, like Duotrope, where all SW from many publishers, are listed. Or compile a list of publishers and publications you want to submit to, and create your own calendar of submissions.
3. Mailing in self-published books, or emailing the PDF of one, is not going to help for the reasons already mentioned. However, it also needs extra clarifications. Publishers rarely take on self-published books already on sale through online retailers, such as Amazon. It can happen, but there is generally a reason for that.
A book that has already been out there simply complicates things:
Pre-orders and sales around the release time are paramount to a book. The buzz, the marketing, the book launch, etc., are all in place to maximise impact on release. You've lessened that opportunity.
As a publisher, I send my catalogues to booksellers for the year to come, months in advance. It helps them plan their acquisition. If your book is already out, you've missed that boat, as space is at a premium in stores, and new books get precedence over back catalogues (unless you are Stephen King, etc.).
Often companies or people buy books to resell privately on eBay or Amazon, at small prices. If your book has been through that process, you've created competition for your own baby.
Unfortunately, there are also people who make pirate copies of books. It happens to all authors. If copies are available, the moment a publisher announces a re-release could prompt these sites to get a head-start.
Use your self-published work (stand-alone or series) as a starting point, but offer something new during a SW.
4. Do not send an unsolicited group email to a list of publishers. I must receive at least 5 of such emails, every month. And every single one had said list in the TO: field, rather than the BCC: field... . And never mind GDPR: some of these addresses didn't look like they were supposed to be in the public domain.
5. Same goes for unsolicited submissions. This is the biggie of them all. You might think it's nothing, but replying to several of these every week, when you run a small press, takes time. It doesn't mean you cannot 'enquire' about something, but please check the website first.
When I receive a submission outside of SW I don't have the time to reply - the website says that too. When Luna started, sending a template reply pointing the way to the submission page, took very little effort. Five years down the line, it's not sustainable. Every email takes time, and a small press doesn't have that luxury. Said people, generally follow up with another email asking if we had a chance to read their unsolicited manuscript, and when are we going to do that, and so forth and so on. It is not what I'd call, the beginning of a beautiful friendship...
To conclude. Give yourself the best chances to make it through the door. Establish a relationship based on respect, trust and commitment. Check the website in the first place, and if you cannot see the answer you are looking for, use the contact form to ask. It's there for that very reason. Socials too.
I love open submission weeks or days - in fact, I wish I could be open all the time. Alas, I must be realistic and take on what I know I can deliver. I know many of my fellow publishers are in the same situation. So don't be upset if you don't get a reply, but do go and check the website. Opportunities will come!