The dovecot plugin uses the dovecot Stats protocol to gather metrics on configured domains. You can read Dovecot's documentation here
# Read metrics about dovecot servers
[[inputs.dovecot]]
## specify dovecot servers via an address:port list
## e.g.
## localhost:24242
##
## If no servers are specified, then localhost is used as the host.
servers = ["localhost:24242"]
## Type is one of "user", "domain", "ip", or "global"
type = "global"
## Wildcard matches like "*.com". An empty string "" is same as "*"
## If type = "ip" filters should be <IP/network>
filters = [""]
server: hostname
type: query type
ip: ip addr
user: username
domain: domain name
reset_timestamp time.Time
last_update time.Time
num_logins int64
num_cmds int64
num_connected_sessions int64 ## not in <user> type
user_cpu float32
sys_cpu float32
clock_time float64
min_faults int64
maj_faults int64
vol_cs int64
invol_cs int64
disk_input int64
disk_output int64
read_count int64
read_bytes int64
write_count int64
write_bytes int64
mail_lookup_path int64
mail_lookup_attr int64
mail_read_count int64
mail_read_bytes int64
mail_cache_hits int64
telegraf --config t.cfg --input-filter dovecot --test
* Plugin: dovecot, Collection 1
> dovecot,ip=192.168.0.1,server=dovecot-1.domain.test,type=ip clock_time=0,disk_input=0i,disk_output=0i,invol_cs=0i,last_update="2016-04-08 10:59:47.000208479 +0200 CEST",mail_cache_hits=0i,mail_lookup_attr=0i,mail_lookup_path=0i,mail_read_bytes=0i,mail_read_count=0i,maj_faults=0i,min_faults=0i,num_cmds=12i,num_connected_sessions=0i,num_logins=6i,read_bytes=0i,read_count=0i,reset_timestamp="2016-04-08 10:33:34 +0200 CEST",sys_cpu=0,user_cpu=0,vol_cs=0i,write_bytes=0i,write_count=0i 1460106251633824223
* Plugin: dovecot, Collection 1
> dovecot,server=dovecot-1.domain.test,type=user,[email protected] clock_time=0.00006,disk_input=405504i,disk_output=77824i,invol_cs=67i,last_update="2016-04-08 11:02:55.000111634 +0200 CEST",mail_cache_hits=26i,mail_lookup_attr=0i,mail_lookup_path=6i,mail_read_bytes=86233i,mail_read_count=5i,maj_faults=0i,min_faults=975i,num_cmds=41i,num_logins=3i,read_bytes=368833i,read_count=394i,reset_timestamp="2016-04-08 11:01:32 +0200 CEST",sys_cpu=0.008,user_cpu=0.004,vol_cs=323i,write_bytes=105086i,write_count=176i 1460106256637049167
* Plugin: dovecot, Collection 1
> dovecot,domain=domain.test,server=dovecot-1.domain.test,type=domain clock_time=100896189179847.7,disk_input=6467588263936i,disk_output=17933680439296i,invol_cs=1194808498i,last_update="2016-04-08 11:04:08.000377367 +0200 CEST",mail_cache_hits=46455781i,mail_lookup_attr=0i,mail_lookup_path=571490i,mail_read_bytes=79287033067i,mail_read_count=491243i,maj_faults=16992i,min_faults=1278442541i,num_cmds=606005i,num_connected_sessions=6597i,num_logins=166381i,read_bytes=30231409780721i,read_count=1624912080i,reset_timestamp="2016-04-08 10:28:45 +0200 CEST",sys_cpu=156440.372,user_cpu=216676.476,vol_cs=2749291157i,write_bytes=17097106707594i,write_count=944448998i 1460106261639672622
* Plugin: dovecot, Collection 1
> dovecot,server=dovecot-1.domain.test,type=global clock_time=101196971074203.94,disk_input=6493168218112i,disk_output=17978638815232i,invol_cs=1198855447i,last_update="2016-04-08 11:04:13.000379245 +0200 CEST",mail_cache_hits=68192209i,mail_lookup_attr=0i,mail_lookup_path=653861i,mail_read_bytes=86705151847i,mail_read_count=566125i,maj_faults=17208i,min_faults=1286179702i,num_cmds=917469i,num_connected_sessions=8896i,num_logins=174827i,read_bytes=30327690466186i,read_count=1772396430i,reset_timestamp="2016-04-08 10:28:45 +0200 CEST",sys_cpu=157965.692,user_cpu=219337.48,vol_cs=2827615787i,write_bytes=17150837661940i,write_count=992653220i 1460106266642153907