- SMTP Errors: Issues related to the Simple Mail Transfer Protocol (SMTP), such as "421 Service not available, closing transmission channel." - DNS Errors: Problems with DNS settings, leading to errors like "550 5.1.2 Bad destination host 'DNS Hard Error'." - IP Blocks: When your IP address is blocked by the recipient's server, resulting in errors like "554 5.7.1 Service unavailable; Client host [x.x.x.x] blocked."