XF 2.1 "503 5.5.1 Error: nested MAIL command" from SMTP server

[nocte]

I use mailtrap.io as SMTP server on my dev environment. Sometimes when multiple conversations are created I get this error back:

503 5.5.1 Error: nested MAIL command

The mailtrap.io support wrote:

The error means that the code is trying to send an email with several “MAIL FROM” commands, which is not valid for SMTP server (there can be only one “MAIL FROM” command for an email message).
Please update the code to include one "mail from" command and let us know if it helps.

May this be a XF 2 bug?