Guild mail is becoming an increasingly important issue in my experience.
Especially when you're a guild leader and want to send mails to your members about 'rules & guidelines', an upcoming event or - if you have a guildcouncil or something similar - about points to discuss in a meeting, etc...
It's just way too much work to copy & paste the text in every new mail you're going to create.
A possibility to address several people at once, by typing the names separated by a semicolon, for example, would be a good start, but if your guild is pretty big, than that wouldn't be sufficient either...
So, what about a scrolldown menu where you can choose from a selection? A lot like how we can do it now, but one which stays open, where you can select options like 'guild' to send mail to the entire guild (only for those with the guild mail rights to prevent other members from possibly spamming the guild mail), and which also shows every toon's name... Then it's just a matter of selecting those you want to send the mail to.. you click an 'okay' button or so, and voilá, you can send to multiple people at once.
(Just some thoughts that many people have probably extensively expressed before, but it's always good to think along with the devs and give them ideas, right?