Smtp server is just not responding

I tried sending email and it just refuses without reason. and after few months, it goes through. why is it unstable? What are the backup options.

Does it give you an error message? If so, copy/paste here.

Thunderbird just says could not send with smtp and it has no reason. After 5~10 mins the msg goes. The webmail is the worse effected. SMTP error does not get recorded in the webmail. Senders using webmail think the msg was send and the other side think the email got deleted. The fact is the email never got sent and webmail failed to report it.