There are two safe modes; Thunderbird's own, and your operating system's. When using the operating system safe mode, you need to choose "Safe mode with networking".

Thunderbird's own safe mode disables add-ons and UI tweaks. It should not interfere with connectivity and we generally suggest it to eliminate possible sources of trouble.

http://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10 https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

But over and above that, who is your email provider? And your ISP?

Duplicate threads https://support.mozilla.org/en-US/questions/1147602 https://support.mozilla.org/en-US/questions/1147595 https://support.mozilla.org/en-US/questions/1147599 https://support.mozilla.org/en-US/questions/1147395

are being closed in favor of the canonical thread 'https://support.mozilla.org/en-US/questions/1147517.