Re: -"masu" or -"mashita"?
doumo really is one of the more versatile words in the Japanese language. If you look it up in the dictionary it lists 6 disperate meanings:
1: (Abbreviation) thanks; (Adverb)
2: much (thanks); very (sorry); quite (regret);
3: quite; really;
5: no matter how hard one may try; (Interjection)
6: greetings; hello; goodbye
When I first came to Japan an Aikido friend,who is incidentally rather active on this site, told me how a senior Aikikai shihan here in Japan once told him that anytime you don't know what to say, just say doumo.