When you plan to be out of office for a longer time you can set up an automatic answer to the incoming emails on you account. To use it you have to install the Thundebird add-on Sieve nightly build.

Once installed you will find a new entry under Tools --> Sieve Message Filters where you can add your scripts.

In the following vacation script example you should write the dates in the format YYYY-MM-DD and replace every <VARIABLE>.

require ["fileinto", "regex", "envelope", "vacation", "date", "relational"];
# rule:[Recognised SPAM]
if anyof (header :is "X-Spam-Flag" "YES")
{
    fileinto "Junk";
  # Stop here so that we do not reply on spams
  stop;
}
# rule:[Vacation]
if allof(currentdate :value "ge" "date" "<START_DATE>",
         currentdate :value "le" "date" "<END_DATE>",
        not header :contains "Precedence" ["bulk", "list"]) {
vacation
  # Reply at most once every 5 days to a same sender
  :days 5
  :subject "Out of office reply"
  # List of additional recipient addresses which are included in the auto replying.
  # If a mail's recipient is not the envelope recipient and it's not on this list,
  # no vacation reply is sent for it.
  :addresses ["<USER@DOMAIN_IOM>", "<ALTUSER@DOMAIN_IOM>", "<ALIAS@DOMAIN_IOM>"]
"I'm out of office from <START_DATE> to <END_DATE>.
Best regards
<SIGNED>";
}