Setting the envelope sender for outgoing messages
Before transmitting the message to the server, the client has to specify the sender and the recipient of the message with the "FROM" and "RCPT" SMTP commands. In order to avoid horrible confusion, you need to bear in mind that the value used during the SMTP session as the sender address (in the FROM command) may be different from the actual addresses used in the mail headers (From, Sender, Reply-To headers all with a slightly different semantic).
The FROM SMTP address is either called envelope sender or envelope from address or return path and it's finally appended as an actual header when the message is finally delivered.
The return path value is a sensible value, because this value will be passed as the next "FROM" SMTP command if the message is relayed by the server (transmitted to another mail server). This address has to be meaningful (e.g. it has to be well-formed and its domain has to correspond to an existing domain) otherwise the message could be filtered out as spam (as many spam messages don't have a valid return path). The return path is also, as the name suggests, the address to which an eventual error message has to be sent, for example in the case of delivery failure.
It could be set to the address of the postmaster of a site (e.g. firstname.lastname@example.org), or it could be set (in the case of a satellite system) to the address provided by the mail account provider (email@example.com) when the message is relayed to an external mail server.
By default most SMTP client agents will set the return path to the local address of the process which spawns them, mutt in our case, so if the user alice in the rabbit-house.example.ORG system will compose and send a message with mutt, the SMTP client to which the message is piped in will use as return path the address firstname.lastname@example.org.ORG(it depends on the mailer configuration).
You can actually force the return path set by the mutt spawned SMTP client with these lines:
set use_envelope_from # a boolean value, you can deselect it with set nouse_envelope_from set envelope_from_address= "alice@beyond-the-mirror.NET"
The manual says about use_envelope_from:
When set, mutt will use ``$envelope_from_address'' as the envelope sender if that is set, otherwise it will attempt to derive it from the "From:" header. Note that this information is passed to sendmail command using the "-f" command line switch, so don't set this option if you are using that switch in $sendmail yourself, or if the sendmail on your machine doesn't support that command line switch.
Fortunately most SMTP mailers (postfix, exim) mimic the sendmail command line interface, so this will work with those mailers too.
You may also want to dynamically set the use_envelope_from and envelope_from_address variables using the usual mutt macro/hook tricks (see ConfigTricks). A simple way to set it dynamically would be to set use_envelope_from and then let mutt use the (eventually dynamically generated) "From: " header address (set with my_hdr or with the from variable).
Note that in order to be able to perform this operation you may need special permission: e.g. with exim if your username isn't in the untrusted_set_sender list it need either to be in the trusted_users or in the trusted_groups list to let you set the return path.
Finally the local server may rewrite the return path (eventually overriding the mua return path settings), that could result useful when using a mua not clever as mutt (e.g. mailx, used in many scripts).