I have had two clients over the years who run their own domains, and wanted me to use an e-mail address within that domain. They also insisted that all of my outgoing e-mail be under their domains, rather than the lerner.co.il domain I've been using for 20 years.
I have all of my GMail forwarded to my regular account, so it's mostly just a bother with sending out messages. Twice in one week, I responded to e-mail from Thunderbird, rather than GMail, and thus ended up exposing the fact that I'm a consultant rather than an employee of the company. The client went totally ballistic on me over this.
I'm pretty sure that there's a way for me to configure Thunderbird and/or Gmail such that I can use Thunderbird as a front-end client to their Gmail domain, but I didn't bother. Mostly, I just found it to be a foolish and controlling way for them to run their businesses, and hide the fact that I'm a consultant (when the simplest search on Google or LinkedIn would reveal otherwise).
You can configure your clients' Gmail accounts in Thunderbird.
When composing an email, pay special attention to the "From:" field. It is a drop down list that shows all the email accounts configured in your Thunderbird. Choose the one you need to use to save your client's face.
A more sophisticated approach is use the Identities feature of Thunderbird.
I have all of my GMail forwarded to my regular account, so it's mostly just a bother with sending out messages. Twice in one week, I responded to e-mail from Thunderbird, rather than GMail, and thus ended up exposing the fact that I'm a consultant rather than an employee of the company. The client went totally ballistic on me over this.
I'm pretty sure that there's a way for me to configure Thunderbird and/or Gmail such that I can use Thunderbird as a front-end client to their Gmail domain, but I didn't bother. Mostly, I just found it to be a foolish and controlling way for them to run their businesses, and hide the fact that I'm a consultant (when the simplest search on Google or LinkedIn would reveal otherwise).