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Publication is necessary for defamation. The post must be intentional or negligent. Here publication does not mean giving publicity but making the statement known to another person who is not the defamed one.
If a defendant has made a defamatory statement only to the plaintiff and no one else, then it will not be considered defamation. If by his own free will he gives it to another person to read it, it will be published by himself and therefore the defendant cannot be held responsible, but when he has a duty to send it to another person, the defendant is held responsible.
In Pullman v. Hill, [(1891) 1 OS 524, the plaintiff dropped a letter in an open card containing defamatory matter, which is likely to be read by somebody else. It was held that it amounts to publication.
In Theaker v. Richardson, [(1962) 1 All ER 229] it was held that sending something by telegram or postcard that can be read by others can be claimed as publication.
In Delacroix v. Thevenot, [(1817) 2 Stark 63]ii It was noted that if a person knowingly sends something in writing to defame a person in a sealed envelope to be opened by someone else, then this amounts to publication.
In Sadgrove v. hole, [(1829) 10 BRC 263] the defendant sent a postcard to a third party containing a defamatory statement in relation to the plaintiff, but the plaintiff’s name was not mentioned, and no stranger unaware of the circumstances would have known who was meant.