Mlsend, mlsend2 and mlflow are domains owned by MailerLite and are used for sending emails. Similar to how Google's UTM tags work, the "ml" domains are used to gather click statistics to support our customers' email marketing efforts.
For example, if you are including a link in your campaign, subscribers will be directed to the intended domain via an mlsend domain in order to properly record data for your reports. If you would prefer the links to redirect using your own domain rather than the default mlsend one, you can do so by setting up domain alignment.
If you are seeing that your emails are being sent with a notice that says “Via mlsend” on the sender’s email address, this is sometimes added in Gmail or in other email providers to indicate that the email is sent through MailerLite’s servers. It doesn’t do any harm, as the emails are in fact being sent via the mlsend domain. Gmail and other email providers will stop displaying this once they start “trusting” the sender, which can be helped by authenticating the sending domain and building a healthy domain reputation.