I did not type in "mailto:" This appeared in the Preview and I do not know where this is coming from.
The "Enhanced Text" mode automatically adds the "mailto:" prefix, which I've also noticed can be quite annoying. This behavior has been around for over 20 years. Consider using different composing modes like Markdown to avoid this.
Regarding the URL issue: the URL plain://webmaster@host-name.org:xyz+PWD@MAILHOST:587 is invalid per the RFC because the "userinfo" part must be percent-encoded to avoid conflicts. In this case, the "@" in the userinfo conflicts with the "@" that terminates the userinfo field. The module should handle this scenario gracefully rather than crashing, and I've now added the necessary percent-decoding for that field in the updated code in the repository.
For instance, if you use
plain://webmaster%40host-name.org:xyz+PWD@MAILHOST:587
the module correctly decodes the user as webmaster@host-name.org (everything before the colon) and the password as xyz+PWD.
Please update.
