how to remove \n from file?

[buran]

instead of writing the file like you do, look at configparser module from standard library. Your code will be way more clean and readable

import configparser

my_config="""[options]
verbose = 1
read_all = false
delete = true
received = false
delivered_to = false
message_log = /home/mail/log
message_log_verbose = true

[destination]
type = MDA_external
path = /usr/bin/exim
arguments = ("-i", "-bm", "user@localhost")
unixfrom = true"""

config = configparser.ConfigParser()
config.read_string(my_config)

cur  = conn.cursor() 
query = "SELECT user, passwd, server FROM pop"
cur.execute(query)
for idx, row in enumerate(cur, start=1):
    credentials = dict(zip(('username', 'password', 'server'), row))
    config['options'].update(credentials)
    with open(f'mail{idx}', 'w') as f:
        config.write(f)

instead of string my_config can be a dict

Also I don't know if MariaDB python connector supports DictCursor like MySQL connector, but if it does - you can use it.