I would like to be able to receive and send mail from my custom domain in thunderbird. I've configured the DNS nameservers to point to cloudflare, and cloudflare has the MX & TXT records setup to use mailgun for sending and receiving mail. Mailgun forwards received mail to my gmail address. I've replace my custom domain name with customdomain.me

Namecheap nameservers config (domain name provider)

bayan.ns.cloudflare.com
gene.ns.cloudflare.com

Cloudflare DNS

Type  Name            Content            Proxy status
MX    customdomain.me mxb.mailgun.org 10
MX    customdomain.me mxa.mailgun.org 10
TXT   mailo_domainkey ...
TXT   customdomain.me v=spf1 include:mailgun.org ~all
CNAME email           mailgun.org        Proxied

Mailgun config

catch_all() Priority: 0
forward("user@customdomain.me")

Domain settings:
DKIM selector     mailo
Tracking hostname email

SMTP credentials
username: postmaster@customdomain.me

I am currently switching from mailspring to thunderbird because of a mailspring bug (see here). In thunderbird, I tried adding an alias to my gmail account but all email sent from the alias ends up in spam because of the mismatch between sender and From field. I also tried setting up a new account with the following settings but this isn't worki

protocol: imap
hostname: imap.gmail.com
port: 993
username: myusername@gmail.com

protocol: smtp
hostname: smtp.mailgun.org
port: 587
username: postmaster@customdomain.me

but thunderbird "fails to find the settings for this email account". I can send from an alias in gmail using the mailgum smtp server and postmaster@customdomain.me username. I waas also able to do this in mailspring.

What am I missing?

Thanks