Q: Corresponding author not in list of main authors

Authors are usually listed in the order of their contribution to the research. Most often, the first author is the one who contributed the most to the research and/or manuscript writing, while the corresponding author is involved in the manuscript submission process and all related correspondence (pre- and post-publication). In fact, the last author might usually be the corresponding author.

Corresponding authors are typically senior researchers, i.e., the group leader or principal investigator. The reasons for this are as follows:

1. Thus, they tend to have considerable experience in the submission and publishing process.
2. Their contact address is less likely to change vis-à-vis an early career researcher.
3. They might be more familiar with journal requirements, ethics committee approvals, clinical trial registration documentation, etc.

If the corresponding author also performed the bulk of the work, he/she might be both the first author and corresponding author. Meanwhile, some papers even have more than one corresponding author; however, some journals discourage this and prefer that only one corresponding author be specified.

The simple answer to your question is that the corresponding author need not be the first author (unless he/she contributed most significantly to the research) but must certainly be in the author list. That is, he/she should fulfill the ICMJE authorship criteria to be one of the authors. If the corresponding author is your group leader, who provided major intellectual inputs and supervision but did not actively perform the experiments or write the manuscript, they may be listed as the last author. If not, the name should appear according to the order reflecting contribution.

You can read about the roles and responsibilities of a corresponding author in detail here: 6 Pre-submission tips for corresponding authors

It is advisable to decide author order in advance to avoid any conflict at later stages. This article discusses how the order of authorship can be decided: Deciding the order of authors on a paper