What we've been doing is changing the mail server in the DFM Options Events & Alerts menu. This is a hack and I don't like this approach much.
In the Setup/Alarms menu, you can disable each alarm but this has to be done invidually and then you have to remember which were disabled before you started so that you can enable the right ones again when your maintenance is done. If you have 1 or 2 alarms this might work for you.
Ideally, on the Alarms menu, I'd like to see either a "disable all alarms" global option or a per-filer setting to disable all alarms for just that filer. Per-filer (per cluster) would be preferable if you had a lot of filers - for example, we've got filers in multiple sites and it would really suck to lose notification for a filer in another city while I've got a local one in maintenance mode.
Short answer - there is no good answer that we've found.