sed

$ cat <<END >> email.dat
> gean@example.com
> william@example.com                                                                                                     
> jeane@example.com                                                                                                       
> thais@example.com                                                                                                       
> thamiris@example.com                                                                                                    
> END

Adicionando um ponto e vírgula “;” no final de cada linha

$ sed 's/$/;/' email.dat 
gean@example.com;
william@example.com;
jeane@example.com;
thais@example.com;
thamiris@example.com;

Para unir todas as linhas

$ sed 's/$/;/' email.dat | sed ':a;$!N;s/\n//;ta;'
gean@example.com;william@example.com;jeane@example.com;thais@example.com;thamiris@example.com;