I want to add a header field to an email retrieved from an imap server and save this change back to the server.
This does not do the job: [email AddHeaderField:@"X-name" fieldValue:@"value"];
Do I have to do more or something else?
Messages on an IMAP server are immutable, meaning they cannot be changed. See this from the IMAP specification (RFC):
The combination of mailbox name, UIDVALIDITY, and UID must refer to a single immutable message on that server forever. In particular, the internal date, [RFC-2822] size, envelope, body structure, and message texts (RFC822, RFC822.HEADER, RFC822.TEXT, and all BODY[...] fetch data items) must never change. This does not include message numbers, nor does it include attributes that can be set by a STORE command (e.g., FLAGS).
Also, if the IMAP RFC is reviewed in detail, you'll find there is no IMAP protocol command for replacing an existing email with different content. In other words, once an email is on the server, it cannot be modified.
The only choice is to fetch the email from the server, make modifications, then upload the modified email to the server using any of the following methods: AppendMail, AppendMime, AppendMimeWithDate, or AppendMimeWithFlags, and then finally to delete the original email from the server.