Dovecot not serving all messages through IMAP

I have a Debian / Dovecot / Postfix standard setup. Additionally I am running notmuchmail indexer to quickly search for messages. As a mail client I use emClient on Windows, I also got Rainloop Webmail running.

I used notmuchmail to search for a particular message and i found it, received around 5 years ago. When I check with emClient, the messages dated back only until 3 1/2 years ago. First I suspected emClient to not have downloaded all messages, but then, checking with Rainloop, I also saw the oldest message visible via IMAP is 3 1/2 years old.

Searching the problem, it is suggested to resyncronize the mail client by deleting the dovecot-uidlist in my Maildir (https://www.dovecot.org/list/dovecot/2013-October/092803.html), which i did. emClient resynced all messages accordingly BUT nothing changed, the oldest message is 3 1/2 years old.

So now i am at a loss… Is there anything else i can do to have access to ALL my messages through IMAP. My Server is running Dovecot 2.2.13.

output “of dovecot -n”

# 2.2.13: /etc/dovecot/dovecot.conf
# OS: Linux 3.16.0-4-amd64 x86_64 Debian 8.9
login_trusted_networks = 192.168.1.0/24
mail_location = maildir:~/Maildir
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope encoded-character vacation subaddress comparator-i;ascii-numeric relational regex imap4flags copy include variables body enotify environment mailbox date ihave
namespace inbox {
  inbox = yes
  location =
  mailbox Drafts {
    special_use = Drafts
  }
  mailbox Junk {
    special_use = Junk
  }
  mailbox Sent {
    special_use = Sent
  }
  mailbox "Sent Messages" {
    special_use = Sent
  }
  mailbox Trash {
    special_use = Trash
  }
  prefix =
  separator = /
}
passdb {
  driver = pam
}
plugin {
  sieve = ~/.dovecot.sieve
  sieve_dir = ~/sieve
}
protocols = " imap sieve"
quota_full_tempfail = yes
service auth {
  unix_listener auth-userdb {
    mode = 0600
    user = username
  }
}
service managesieve-login {
  inet_listener sieve {
    port = 4190
  }
}
ssl = required
ssl_cert = </etc/letsencrypt/live/a.b.com/fullchain.pem
ssl_cipher_list = AES128+EECDH:AES128+EDH
ssl_dh_parameters_length = 2048
ssl_key = </etc/letsencrypt/live/a.b.com/privkey.pem
ssl_protocols = !SSLv2 !SSLv3
userdb {
  driver = passwd
}
protocol lda {
  info_log_path =
  log_path =
  mail_plugins = " sieve"
}