It's look like mIRC's problem, first
/identd off <identd> doesn't change the identd on options as it should, second
/server -m irc_server -i nickname alternative_nickname emailaddr realname seems that emailaddr doesn't work.
So the only way to do it that using ON START event is this but is incorrect as mIRC has problem or the working way is different that i read on help file and help file seems outdated on that part, until someone who knows better will answer use that code:
ON *:START: {
.identd on emailaddr1
.identd off
server -m server1 -i nickname1 alternative_nickname1 emailaddr1 realname1
.identd on emailaddr2
.identd off
server -m server2 -i nickname2 alternative_nickname2 emailaddr2 realname2
.identd on emailaddr3
.identd off
server -m server3 -i nickname3 alternative_nickname3 emailaddr3 realname3
}