E-mail aliases have been created through the My Services form. Identical aliases have been created in Thunderbird mail client with the alias entered for both Email Address and Reply-to Address. Mail sent from the client using an alias has the original (un-aliased) address in the Reply-to field of the message header when received at the destination. This defeats the purpose of using an alias.
Welcome to the forum.
The community would like to assist, but could clarify a few items for us on your question?
1. From the My Account and under manage email account, what does your email UId display, the alias name you have or original name?
2. Have you try using another email client to test your email account?
1. The original name is displayed for the account.
2. No. The original configuration of Postfix MTA and Thunderbird MUA worked as expected with aliases (messages sent and received under alias name). The MTA has now been changed to comcast business and all mail being sent under an alias is received under the alias name but the 'reply to' defaults to the original account name.
Thank you for the repost.
Based on it, I suggest setting the email account another email client or another computer for comparison test.
This could be cache or setting error on that computer, and with the test it will assist in the troubleshooting.
Please post to community of any update to this issue, we look forward to it.