Auch wenn es sich für dich komisch anhört: Es heißt wirklich "Dear Sir or Madam" - ein der Einzahl. Ich meine auch, das dies britisches Englisch ist. Eine amerikanisch-englische Anrede wäre "Ladies and Gentlemen".
Viel Erfolg bei deinem Brief. Gruß Anna

Ok, vielen Dank! Die Mail geht an eine französiche Firma, der Fauxpas wäre wahrscheinlich gar nicht aufgefallen. Aber man muss es ja nicht drauf anlegen ;-)

"Dear Sir or Madam" is for a single person whose name I've not been able to find out.

Personally, if it fits, I would either write "Dear Ms Jones, Mr Bloggs and Ms Richardson," or write "Dear Ms Jones" and then mention the others later. If you are sending one e-mail to several recipients, at different e-mail addresses, then perhaps "Dear customers", "Dear everyone" (informal) or something similar. If none of those is possible, then the plural is "Dear Sirs and Mesdames" - it is very formal and normally used when you don't know people's names, but in this case it would probably be OK too.

As you said "Sirs and Madams" is a definite no-no. (Madams are women who run brothels.)

Dear Sirs and Madame,

Also soviel ich mitgekriegt habe (lebe in London) kann man ruhig schreiben dear Sirs,
aber niemal dear Madames. Von daher gehe ich davon aus das die oben angegeben Schreibform die richtige ist...

Dear Sirs and Madam,

Also soviel ich mitgekriegt habe (lebe in London) kann man ruhig schreiben dear Sirs,
aber niemal dear Madames. Von daher gehe ich davon aus das die oben angegeben Schreibform die richtige ist...

Ich habe immer gelernt, dass man einfach "Dear Sirs" schreibt, wenn man den Empfänger nicht kennt. Nur wenn man 100 % sicher ist, dass es in der Firma nur Frauen gibt, kann man "Dear Madams" schreiben..

The plural of Madam (unless you're talking about brothel-keepers) is Mesdames. This is also suitable for a letter (or was, when I was young).

As someone said above "Madams" is absolutely out!!!

Why don't you listen to the native speaker?

CM2DD said everything there is to say.

I will summarise:

1) Do not use: "Dear Sirs and Madams", as this translates into "Sehr geehrte Herren und Puffmuttern".

2) "Dear Sirs is fine, if you are absolutely sure that there are no female recipients.

3) "Dears Sir or Madam is correct in every other context, where you do not address the recipients by name- each letter will go to one reader, who will be then be addressed either as "Sir" or "Madam".

4) "Dear Sirs and Mesdames" would be the correct pluralisation, if you absolutely must use it- however, it is a little old fashioned and rarely used these days.

5)"Dear Sirs and Madame" would only be acceptable, if one addressed several gentlemen and one French lady. In English, the correct singular is 'Madam', without the 'e' at the end.

6) As for "Dear Madams", see point 1 above

*remove one 's' from point 3*