Alpine SMTP error 334: "Can not authenticate to SMTP Server"

Alpine SMTP error 334: "Can not authenticate to SMTP Server"

[This is a follow-up thread to https://help.zoho.com/portal/en/community/topic/alpine-smtp-error-421-error-sending-can-not-authenticate-to-smtp-server-421-smtp-connection-broken-reply]

I've not seen the 421 error for some time now.

However, recently when sending email (usually when replying to received mail), I'm intermittently hitting error 334 on the first attempt to send the email. Retrying a second time to send the email succeeds. I've not altered my settings, they are still:

smtp.zoho.com:465/user=john@mydomain.ca/ssl/submit

Any suggestions?