My paper was rejected a few hours after submission with the following explanation:
"Regretfully, your manuscript is being Immediate Rejected without review since it is a short correspondence item while we do not publish correspondence. "
What does this mean? Is my manuscript too short?
Many journals publish correspondence or short reports that are brief research works, usually with a single finding, straightforward methods, and not much more than that. They are intended for quick, "Hey, we've always wanted to know the value of X, and turns out it's 7" studies - they belong in the literature, and may be useful, but are not a full research paper.
They are often also referred to as letters, notes, or by other names. Sometimes they're purely defined by concept, sometimes by word count.
For example, consider Ecology:
"Notes are short papers that present significant new observations and methodological advances. Notes may contain results that are not sufficiently elaborated or developed as to justify an Article, but which are still of considerable potential significance."
vs.
"...an Article tells a more complicated story with distinct components. The greater length of Articles relative to Reports must be justified by their greater complexity
Or American Journal of Epidemiology, which uses word counts:
The maximum number of words per article, exclusive of tables, figures, references, and abstract, should be as follows: Original Contribution, 3,500; ...Brief Original Contributions, 2,000 (with no more than 2 half-page tables and 40 references)
or Infection Control and Hospital Epidemiology:
Original Articles should include a title page, a structured abstract of no more than 250 words (see below), a text of no more than 3,000 words, no more than 7 tables and figures, and no more than 40 references.
Concise Communications should include a title page, a narrative abstract of no more than 50 words, a text of no more than 1,200 words, no more than 2 tables or figures, and no more than 10 references.
Research Briefs should include a title page, a text of no more than 900 words, no more than 1 table or figure, and no more than 10 references. This category of article is intended for the presentation of short, focused, and evidence-based experimental observations: substantial preliminary and novel results of importance to the journal readership but not substantial enough in content to warrant a longer presentation. Research Briefs undergo the same peer review as longer article types.
Your paper is either too short, or only presents a short, focused result that the journal does not consider a "full" paper. It's hard to know, as they won't exactly lay out the definitions of papers they don't accept, but you may want to look at similar journals to see if there is a field-based consensus for what a sufficiently large finding is.
Fomite already gave an excellent general answer, but I may be able to add some specific information if your submission was to one of the IEEE Transactions (rejection from which would put you in fine company).
Some of the Transactions indeed used to publish "Correspondence Items"; from the Transactions on Signal Processing Author Guide dated October 2009:
Correspondence items are short disclosures with a reduced scope or significance that typically describe a use for or magnify the meaning of a single technical point, or provide brief comments on material previously published in the TRANSACTIONS.
However, in the current Author Guide, this category is absent and replaced by the "Comment Correspondence", which is a different beast:
Comment Correspondences provide brief comments on material previously published in the TRANSACTIONS.
Presumably, this is due to the fact that there is now a dedicated journal, the IEEE Signal Processing Letters. However, this has a different scope:
The IEEE Signal Processing Letters is a monthly, archival publication designed to provide rapid dissemination of original, cutting-edge ideas and timely, significant contributions in signal, image, speech, language and audio processing.
Note that in contrast to a Correspondence Item, a "significant contribution" is still required, and that in addition (and in contrast to the Transactions) this needs to be on such a hot topic that by the time the normal reviewing process has finished, there would be significantly less interest in it.
(If you did not submit to IEEE, this may still be a useful indication how another discipline handles this type of publication, and underline the fact that there are several different categories of short communications.)
