library(dplyr)
library(tidyr)

dat <- data_frame('date' = Sys.time(),
                  'from' = c("person1@gmail.com", "person2@yahoo.com",
                             "person3@hotmail.com", "person4@msn.com"),
                  'to' = c("person2@yahoo.com,person3@hotmail.com", "person3@hotmail.com",
                           "person4@msn.com,person1@gmail.com,person2@yahoo.com", "person1@gmail.com"))

dat %>% separate(to, into = paste0("to_", 1:3), sep = ",", extra = "merge", fill = "right")

#Source: local data frame [4 x 5]
#
#                 date                from                to_1                to_2              to_3
#               (time)               (chr)               (chr)               (chr)             (chr)
#1 2015-10-22 14:52:41   person1@gmail.com   person2@yahoo.com person3@hotmail.com                NA
#2 2015-10-22 14:52:41   person2@yahoo.com person3@hotmail.com                  NA                NA
#3 2015-10-22 14:52:41 person3@hotmail.com     person4@msn.com   person1@gmail.com person2@yahoo.com
#4 2015-10-22 14:52:41     person4@msn.com   person1@gmail.com                  NA                NA

n_vars <- dat$to %>% str_split(",") %>% lapply(function(z) length(z)) %>% unlist() %>% max()

library(splitstackshape)
cSplit(dat, 'to', ',')

library(dplyr)
library(tidyr)

dat %>% mutate(to = strsplit(to, ",")) %>%
        unnest(to) %>%
        group_by(from) %>%
        mutate(row = row_number()) %>%
        spread(row, to)

Source: local data frame [4 x 5]

                 date                from                   1                   2                 3
               (time)               (chr)               (chr)               (chr)             (chr)
1 2015-10-22 15:03:17   person1@gmail.com   person2@yahoo.com person3@hotmail.com                NA
2 2015-10-22 15:03:17   person2@yahoo.com person3@hotmail.com                  NA                NA
3 2015-10-22 15:03:17 person3@hotmail.com     person4@msn.com   person1@gmail.com person2@yahoo.com
4 2015-10-22 15:03:17     person4@msn.com   person1@gmail.com                  NA                NA