Fastmail uses an app password. There is no way on the SMTP tab to enter the app password so – sending mail fails
[SMTP] Unexpected reply from the server: ‘+ eyJzY2hlbWVzIjoiQmVhcmVyIiwic2NvcGUiOiJodHRwczovL3d3dy5mYXN0bWFpbC5jb20vZGV2L3Byb3RvY29sLXNtdHAiLCJzdGF0dXMiOiI0MDAifQ==’.
FastMail uses oAuth rather than a username/password in eM Client.
To add the account go to Accounts > Add Account and enter the email address in the automatic setup. You will be passed to your web browser to enter your credentials (use account password there not app password) and then back to eM Client where the account will be setup.
With oAuth accounts, don’t change any of the setting afterwards. For example as there is a single oAuth token for both IMAP and SMTP, you can’t change the SMTP to a different server or account for sending. If you want to use a different account for sending, setup the account manually using Accounts > Add Account > Mail > Other. Full instructions for both automatic and manual setups are in the Documentation (F1) under Getting Started.
That’s nice, so why am I getting an error message instead of a successful send?
Apparently the SMTP defaults are not correct for fastmail. It works on 587 with force SSL/TLS
Correction, NOT fixed. Still can’t send