As we review poems submitted to the Poets Online monthly issues, we are always in receipt of poems that do not follow our submission guidelines. (see those at poetsonline.org/submit.html) There are very simple items (title in caps; subject line "submission"), technical ones (single-spaced plain text) and optional ones (do you want your name linked to an email or website). There are also ones that will eliminate your submission immediately, such as sending multiple poems, not addressing the current prompt or putting the poem as an attachment.
These are not unusual and every publisher has their own take on guidelines. Some publications are very strict about guidelines. We wouldn't reject a poem because the title was not in capital letters, but some publications would reject work that is in the wrong font or size.
The point is that you need to READ THE SUBMISSION GUIDELINES for every submission, and you should also look at what poems that publisher has used in the past.
I am part of a group that reads submissions for a print magazine and for a manuscript award. With manuscripts, the guidelines are more complex and more specific to the publisher. For example, they might want a short bio OR they might want no identifying information because the submissions are being read blind.
There are no guidelines that apply to all publishers, but if you are preparing your first manuscript, here are some general suggestions that might help you get the document in shape.
For poetry, single-spaced (with allowances for special typography and layout), each poem beginning on its own separate page in the manuscript;
Standard letter size (8.5 x 11") in portrait orientation and use Times, Arial or Helvetica at 12 pt. black font on white page background;
Most publishers want a Microsoft Word .doc or .docx. Some accept a .pdf or .rtf document.
You should have a cover/title page indicating the manuscript’s working title. Usually, that will also provide the author’s full name, residential address, and contact information (e-mail, perhaps a phone).
You might have acknowledgements page of places where some of the poems have been published before.
You need a Table of Contents listing the poem titles and page numbers. This might be the most difficult thing to set up in Word as publishers want page 1 to be the first poem and not the cover or any of the pages prior to the first poem. This can be very confusing to do. The sequential pagination is usually in the upper-right corner (document header) or in the footer. Some publishers ask for the header to include the author's last name and the manuscript title but, as noted earlier, another publisher may want no identifiers.
It seems like the majority of publishers are suing Submittable or an electronic submissions system. This can make things easier for poets and publisher but these services are rather costly (which is why the trying-to-stay-free Poets Online still uses old-fashioned email submissions). Submittable is free for an author to use because the cost is picked up by the publisher. But it's not always free to submit your poems. A submission fee to submit individual poems, manuscripts and enter contest has become almost standard. Fees I have seen recently range from $2 to $35. There are still free submission sites and those range from Poets Online to The New Yorker, Submittable allows you to filter sites that are still free, but those fees often support the actual publications (especially print publications) and awards or payments for acceptances and staff salaries.
One last general thing that really annoys a publisher - if you do simultaneous submissions and your work is accepted, be sure to notify the other publishers to withdraw your submission from consideration. I have been surprised and a bit angry about the number of poems we accepted for a print publication where the poet then told us it was previously accepted or even already published elsewhere! This is an easy to do courtesy. Some publishers have long memories for such indiscretions.