Presumably all unencrypted email is available to federal agencies including IP addresses, header information, and email content (eg, through SMTP with or without SSL and TLS -- thunderbird has an SSL/TLS option so it is not clear whether TLS will be used anyway, depending on the mail server). If PGP encryption is used, then this will be known to such organizations, and since header data is not encrypted, this will likely attract suspicion and increase monitoring on nonencrypted data and other traffic from that IP address.
What are options (general or specific) to use end to end secure email without agencies knowing that emails are being sent? Would VPN with SMTP not work because the emails will become accessible after leaving the VPN server? It seems that web-based email through a VPN (eg, protonmail) would meet these criteria. Are there other options?