[SunilNagpal]

Let total no.of mails =  't'
Mails sent on first day  = 'x'
No.of mails sent more than the previous = 'k'
No.of days, mails were sent  ='n'

nth term = 'x+(n-1)k'
Sum of n terms  totals down to total no.of mails sent i.e 't'
t = (x)+(x+k)+(x+2k)+(x+3k)+...+(x+(n-1)k)
t = (nx )+ k.n.(n-1)/2

12000 = 50x + 10.50.49/2
12000 = 50x + 12250
    50x = 250
        x = 5

Thanks Raj for the structured and universal approach!