SMTPAuthCustom / SetSMTPAuth doesn't appear to be effectual

[james-d-elliott]

Description

When using a custom auth implementation it's completely ignored. By the look of it this is caused by changes to make this thread safe.

Before: https://github.com/wneessen/go-mail/blob/v0.5.2/client.go#L1167-L1171
After: https://github.com/wneessen/go-mail/blob/v0.6.1/client.go#L1312-L1316

Note how c.smtpAuth is never used in the function, and it's value is only ever set or checked if it's nil, there is no other read action on c.smtpAuth.

Value being set: https://github.com/wneessen/go-mail/blob/v0.6.1/client.go#L511-L537

Only read action: https://github.com/wneessen/go-mail/blob/v0.6.1/client.go#L511-L537

To Reproduce

1. Create a new client using NewClient and store it as variable client
2. Implement a custom auth mechanism and apply it via client.SetSMTPAuthCustom
3. Attempt to send an email to a server that requires authentication using client.DialWithContext(context.Background) and client.Send(msg)

Expected behaviour

The auth type should be respected.

Screenshots

No response

Attempted Fixes

No response

Additional context

No response

[wneessen]

Thanks for reporting this James, I'll have a look.

[wneessen]

I've fixed the issue with #429. I've also added corresponding tests, so this doesn't get overlooked again. Sorry for the inconveniences. Once you confirm the PR, I'll get a 0.6.2 release ready to fix the regression.

[james-d-elliott]

Never a worry, that's why we have a full integration test suite on our end, it catches things like this. Will test it now.