[exim] Unable to send the mail but able to receive them

From my mac mail client I've connected to remote exim+dovecot server. Since few weeks back I was not able to send mail but able to receive.

exim in /var/log/messages says:
Jul 14 15:22:15 RPi3B exim[3576]: 2019-07-14 20:52:15 dovecot_plain authenticator failed for ([xx.xx.xx.xx]) [yy.yy.yy.yy]: 535 Incorrect authentication data (set_id=necktwi)

dovecot in /var/log/dovecot.log says:
Jul 14 19:40:27 auth: Debug: client in: AUTH 1 LOGIN service=smtp rip=yy.yy.yy.yy lip=xx.xx.xx.xx nologin resp= (previous base64 data may contain sensitive data)
Jul 14 19:40:27 auth: Debug: client passdb out: CONT 1 VXNlcm5hbWU6
Jul 14 19:40:27 auth: Debug: auth client connected (pid=0)
Jul 14 19:40:27 auth: Debug: client in: AUTH 1 PLAIN service=smtp secured rip=xx.xx.xx.xx lip=yy.yy.yy.yy nologin resp=aeVja333aQeuZWerdHdpAeRecmezaG9lcw== (previous base64 data may contain sensitive data)
Jul 14 19:40:27 auth: Debug: policy(necktwi,xx.xx.xx.xx): Policy check action is continue
Jul 14 19:40:27 auth-worker(11866): Debug: pam(necktwi,xx.xx.xx.xx): lookup service=smtp
Jul 14 19:40:27 auth-worker(11866): Info: pam(necktwi,xx.xx.xx.xx): pam_authenticate() failed: Authentication failure (Password mismatch?) (given password: xxxxxxx)

(log is obfuscated)

Is it the problem with dovecot or exim?
Should exim contact dovecot prior to sending the mail?
I have also tried windows mail client with same error.

... Necktwi