The maildir has a specific format of the files laying underneath, e.g. 1549464014.9224_427.hostname:2,RS. In Debian environment copying this file over SSH without renaming is impossible or I don't know how. The question is how?
NOTE: no renaming should be involved!
The example output:
[user@hostname ~]$ scp '1549464014.9224_427.hostname:2,RS' remote.server.com:/dest/dir/ 1549464014.9224_427.hostname:22: [Errno -2] Name or service not known
The solution with quotation like in scp error while attempting to copy files is not useful.