The two default ports for SMTP are 587 and 465. See which one is set up for you in TB, and try the other. Note that by default port 465 uses STARTTLS for security, while port 587 uses SSL/TLS. If you need assistance with how to check this, then let us know what version of TB you are using.

some mail server have slightly different set of server-address for POP3 & IMAP, so use correct ones.

if you're accessing emails via IMAP protocol, then use only their IMAP+SMTP servers for inbound+outbound. example: AT&T: imap.mail.att.net & port 993 , smtp.mail.att.net & port 465.

if you're accessing emails via POP3 protocol, then use only their POP3+SMTP servers for inbound+outbound. example: AT&T: inbound.att.net & port 995 , outbound.att.net & port 465.

so, in many cases, ISP/ESP/MSP will/may use two different set of SMTP mail server-address.

to increase communication security further, many email/mail service providers are adding more secure settings, may be for your area/location, your ISP/MSP may have switched from: "Normal Password" based authentication thru encrypted TLS/SSL connection, into: "OAuth2" based authentication thru encrypted TLS/SSL connection. etc, etc.

sometime, in some mail server, their IMAP or POP3 uses TLS/SSL encryption-security, but their SMTP uses STARTTLS encryption-security.

THUNDERBIRD = TB

inside TB, if an email account was first created for POP3, then you cannot use IMAP server in it, (unless server-address for both POP3 & IMAP are exactly same), similarly, if an email account was first created for IMAP, then you cannot use POP3 server in it, (unless server-address for both POP3 & IMAP are exactly same). For example, if you have POP3 account (eml-adrs@example.com) now, to use it via IMAP: change email-adrs in POP3 account into eml-adrs-1234@example.com a temporary non-existing eml-adrs, then create IMAP account with correct eml address eml-adrs@example.com & IMAP protocol. so similarly, if you have IMAP account (eml-adrs@example.com) now, to use it via POP3: change email-adrs in IMAP account into eml-adrs-1235@example.com a temporary non-existing eml-adrs, then create POP3 account with correct eml address eml-adrs@example.com & IMAP protocol. Use either IMAP or POP3, don't enable both at same time in same email-client software, don't use same eml-adrs in two types of protocol (IMAP/POP3) based account in TB, use correct eml-adrs in one account which you will use, So change eml-adrs slightly in other one & disable that account.

Provide more info on your settings that you're using, TB version number, etc, etc.