After a publisher has had a manuscript for a long period of time, what is the best way to check on the status of the manuscript? Is it acceptable to email the publisher if an email address is available? Is it best to send a letter by snail mail? Or, is it best to call and ask about the status?
Do whichever it says in their submission guidelines.
If their submission guidelines do not address when and how to contact them after your manuscript has been submitted, then I would use e-mail if an address is available. That gives them the ability to respond at their convenience.
Second choice is a note, but include your e-mail address so that they may respond to you quickly, easily and at no cost to them.
Last choice is a phone call. I don't like it when someone calls to check up on a manuscript that I've had for a "long period of time" because I'm already feeling guilty that I've made them wait. Most often, the reason I'm late is because I've been unusually busy. Then if they call when I'm already out of my mind with overworkedness, I'm distracted, I get flustered and I can't find their manuscript in my pile of stuff, and ... Avoiding this real-time conversation is the reason e-mail was invented, wasn't it?