The problem with having a subscribe button without the word 'subscribe' is that there is no standard icon for this.  The only subscribe icon I can think of is the RSS feed subscribe button:  

which obviously wouldn't work in your case.  

What you are describing sounds more like a "Join" situation.  Have a look at other companies that offer services for a monthly fee (ex: Zipcar, Adobe Creative Cloud).

To answer your question though, there is no standard icon for this, so you should probably include the word 'Subscribe' or 'Join' wherever the button appears if you can, because it's just not something people will recognise if it's just an icon.  If appropriate you could use the shopping cart icon, but this doesn't usually make sense unless you offer multiple products, and may be confusing for something you pay monthly for.  If you happen to include an icon along with the text, and do this consistently, it may be OK to use just that icon in some places.

If this is an important call to action (if it's the way they buy your product, it should be) you need to make sure it's as clear as possible. 

Appropriate Icon for a Subscribe or Subscription
I have an e-mail server powered by Postfix. Aside from traditional e-mail accounts on my server, I want to maintain some accounts whose only role is to forward all incoming mail to differnet address on different mail system.

For example. I own domain example.com and I set account with address username@example.com on my server. And I want this one account to forward all incoming messages to mailbox in different mail system, in domain that does not belong to me username@stranger.net.
I know, that i can add an entry in in my virtual_alias_maps lookup table.

username@example.com   username@stranger.net


But I'm worried that to deliver message, Postfix will use original envelope sender address. It's okay if sender is from domain example.com that belongs to me, but otherwise stranger.net server may classify this message as a spam.  

Is there some way, to alter envelope sender on delivery ?  

Are smtp_generic_maps ment for this ? 

If so, is it okay, that my server sends an e-mail using specific envelope sender, and then something else in message being forwarded ? 

How else to deal with such problem ?
