Generally I would recommend for a Teams App to make use of the Teams modalities such as:
Note: If you send a message , and the user does not open the Teams client at all, then the Teams backend will automagically send an e-mail reminder to that user, without you needing to do a thing.
If you still need/want to send e-mails to each member of the Team/channel - you will need to :
or get list of users in that Team
Each Team has an email address for its members--not the email address that sends email INTO the Teams environment and posts it in the channel, but an email address for the members. It is not readily viewable in Teams (I think there's a way, but I don't see it now) but you can usually guess it--it's the Team name mashed together without spaces (e.g. the "IT Management Folks" team is probably [email protected]. It's easy to see in the Teams (or Groups) Admin Interface in Microsoft 365, so your admin can tell you if you can't guess. See, Teams are also Microsoft 365 groups, so they can be emailed like any group.
I've been looking for some way to use the email adress of the M365 group that supports a Team as a Distribution List. Realized I could do this in AAD, just change the setting on the "Group write back state" to "Distribution".
group write back state on AAD
Having searched high and low for an answer to this, but today I have found the relevant settings to enable you to effectively treat an MS Teams team as a distribution list. If you go to the Exchange Admin centre -> Groups -> Microsoft 365. From here, find the group name and then edit the settings. Then set the checkboxes as shown. The key to this is "Send copied of group conversations ..." as this will expand the Team email to the email addresses of its members.